There are a couple of tutorials for creating "Things in a bottle" floating around, but this is my approach. It's a relatively simple process that's designed to produce a high-quality prop from cheap, readily-available materials.
First off, you're going to need a "thing" to bottle. For this example I'll be using a tissue sample from one of the unknown creatures unearthed by the Miskatonic University Antarctic expedition of 1930. This particular chunk o' guts, tentatively identified as a ganglion-like structure, was made from an incredibly cheap and ugly Christmas ornament and some liquid latex:
The ornament's original coating of purple glitter has been removed with some soap and water. The green tissue was created by mixing liquid latex with some apple green craft paint and then applying it to the central core of the ornament using a cotton swab. You can find liquid latex in most craft shops in the sculpting section, where it's sold for making molds. If you can't find it there, don't worry. Just drive to your nearest home improvement store and pick up some liquid latex carpet adhesive or seam sealer, which just so happens to be a very thick version of liquid latex. If you're not sure you have the right kind open the cap and gently waft the fumes towards your nose- if it reeks of ammonia it's liquid latex.
Oh, by the way- DON'T SNIFF THE BOTTLE DIRECTLY. The high concentration of ammonia in the product is hideously foul and potentially dangerous. This is the kind of stuff you want:
With this stuff alone you can create all kinds of nasty looking tissue samples. Experiment with squeezing a blob onto a glass or ceramic plate, smearing it around, and then letting it dry. It'll turn into a rubbery, light yellow solid that readily sticks to itself. Use a sponge to evenly spread the liquid latex around and you'll create a sheet that, when dried, can be rolled up to create a very convincing umbilical cord or veins. Try mixing in some acrylic paint to color it. Try rubbing holes in a dry sheet and then applying more latex. Try sprinkling bread crumbs or coffee grounds into the wet latex. Just keep experimenting and you'll discover all kinds of nifty textures and effects that you'll recognize from dozens of horror movies. It's awesome stuff.
Er..unless you're allergic to latex, in which case it can kill you. Not sure if you're within the minuscule percentage of the population that's allergic to latex? Hit Google and find out how to do a spot test.
Now that you have something to bottle you'll need...uh...a bottle. I picked this one up at the Salvation Army for 50 cents:
You can download the Miskatonic University label over here. Print it, fill it out with a pen with waterproof ink, trim, and then stick it to the bottle with adhesive from a glue stick. I chose to age the paper for the label before cutting it out and gluing it on, but that's a matter of personal preference. Both the label and the cork were aged using a solution of walnut ink crystals, because the greenish tint in the resulting brown stain makes it look all dirty and nasty.
Now we come to one of the little tricks that most tutorials leave out- aging the label. We've already "aged" the paper by staining it, but now we have to reproduce the physical results of years of wear and tear. That means going over the entire label, with particular attention to the edges, with fine sandpaper. I used 320 grit wet/dry paper.
Compare the "before" picture above with this one "after" the sanding. The label looks really old:
Now it's time to stick our tissue sample in the bottle, add our preservative liquid, and seal up the cork with wax.
To keep any unwanted algae from growing inside the bottle I'm going to use rubbing alcohol, specifically 91% isopropyl alcohol, as my fluid. Since I want this bottle to look really, really old I'm going to tint it with food coloring and add some fine powder to give it a really cloudy, grungy look. In a measuring cup I pour two cups of rubbing alcohol, add two drops of green food coloring, and then one drop of red food coloring. That gives me a wonderfully foul greenish brown fluid. To this I add a pinch of baking cocoa.
That's right, cocoa. It's cheap, safe, and the manufacturing process used to make it produces an ultra-fine brown powder with a multitude of uses for aging and grunging up items.
After doing a test fit of my cork I pour enough of the alcohol into the bottle to come up about 5mm short of the cork. Then I'll put the cork in the microwave for about 30 seconds to help drive off any moisture that might be inside it. Why? Because the wax seal on the bottle has to be absolutely air-tight or the alcohol will evaporate in a week or two. Then I set the cork aside and start melting my wax.
Here's something you need to know before moving on to this step- NEVER, EVER HEAT WAX DIRECTLY ON A STOVETOP. Do that and there's a good chance you'll burn your house down. You need to use a double boiler. If you're not familiar with what a double boiler is use Google to find out.
I used an empty tuna can sitting inside an old beat-up pot filled with about an inch of water to melt my wax. The water keeps the temperature inside the can from going over the boiling point of water, so the wax gently melts without vaporizing. Almost any kind of wax will do, from old crayons to candle stubs, but I was lucky enough to score a block of candlemaker's wax from the Goodwill store for a buck. I broke up the block with a sharp knife and placed the pieces in the can, put the can in the water bath, and then turned on the stove. Once the water started to boil I turned the heat down to simmer and waited for the wax to melt.
Once it was liquid I used a cheap craft brush to apply a thin layer of wax all over my cork. Then I placed the cork, resting on a bed of paper towels, inside a microwave oven for about 30 seconds on high in order to liquefy the wax and get it to fill all the pores in the cork. This is an absolutely essential step in order to keep the fluid inside your bottle from evaporating. Once the wax has cooled a bit place the cork in the mouth of the bottle and start brushing on the liquid wax from your double boiler. Take your time and thoroughly coat the cork, the lip of the bottle, and the top of the bottle. Once you've built up a solid layer of wax you can invert the whole bottle and start dipping it into the liquid wax to build up the seal faster.
Here's what it will look like when your done:
Notice how the coloration of the water makes it difficult to see what's inside? Use that to your advantage. If you have a "thing" that's incredibly detailed use pure rubbing alchohol as your fluid so you can show it off. If your "thing" isn't a work of art for the ages use a colored fluid to hide it's details, or lack thereof. Here's a picture of my "ganglion" taken with a flash to make it more visible:
Click through for a higher resolution version. The "ganglion" looks a lot better than I thought it would, so I probably could have used pure alcohol to help show it off. That's a bit of a moot point, however, since I wanted to recreate the look of a really old, really foul sample bottle. That means there's one step left- the final aging treatment. This will be an overall coating of dust and grime created by spraying the entire bottle down with matte finish, drybrushing it with baking cocoa, and then applying another coat of matte finish to hold everything in place.
Here are the results. I switched to a white background to help bring out the details.
Now that it's finished there are a couple of things that, in hindsight, I would have done differently.
First, less food coloring for the fluid. A single drop gives a very intense tint to the alcohol, so next time I think I'll drop the dye onto a piece of paper towel and then dip the paper into the alcohol. That should give me better control over the amount of coloration.
Second, I would have skipped adding the cocoa to the fluid. The matte spray on the bottle surface provides enough cloudiness that adding solids to the alcohol was redundant.
Third, more dust and grime. I might go back and dirty up the wax a bit more by brushing on a paste of cocoa of paint and then wiping it off.
Finally, I need to be less anal about neatness. While I was sealing up the cork with wax a bunch of it dribbled down along the bottle. After going to the trouble of scraping it all off I realized it probably would have looked better with the wax left alone. It would have provided some texture as well as giving the final dusting treatment some crevices to nestle in.
All in all I'm pretty happy with the final results. More importantly, after seeing how you can get good results with a minimum of effort I hope you'll at least consider making one of your own rather than paying an ungodly sum for one on Ebay.
Addendum: In the vast majority of cases it isn't necessary to do anything more than what's already here, but if your specimen bottles are going to be moved frequently, or sent through the mail as a gift, I would strongly recommend that you apply a layer of silicone sealant to the bottle top before adding the wax seal. First, whether you're using a cork or conventional screw-top, apply a bead of silicone along the gap between the top and the glass. Then use a cheap craft brush or toothpick to smear a thin layer all over the top. Wait for the silicone to cure and then apply the wax as normal.
This accomplishes two things. One, it significantly increases the integrity of the lid's watertight seal. Two, it provides enough flexibility between the container, lid, and the wax that changes in temperature or air pressure won't crack it. Some minor fissures are inevitable, and even desirable from an aesthetic standpoint, but any major breaks will compromise the long-term integrity of the preserved specimen and it's fluid.