Tap a cork in a bottle, maybe add a little sealing wax, and you've got yourself a potion.
That's pretty much it for most prop potion bottles. Nothing wrong with that, of course. It's quick, easy, and serviceable.
If you'd like to go a bit beyond those basics you need to check out Mark C. Kehoe's excellent article on Medicine Containers Used In the Golden Age of Piracy. It's aimed at pirate reeneactors, but you'll find a treasure trove of information for propmakers interested in reproducing fantasy potions, elixers, and preparations.
He goes over the types of containers, from glass bottles to oddments like the Burras pipe, used to hold various medical preparations in historic medicine chests. The section I found particularly interesting was the discussion of classic closures. It's filled with the kind of minutia that can help ground a prop in history. As one example, the 19th century chest pictured below demonstrates an interesting historical oddity. Can you spot it?
It's the corks. Tapered, machine cut corks are a relatively modern invention. Until well into the 1800s corks were all trimmed to fit by hand. It's not a huge detail, but taking the time to facet a modern cork should realistically be part of the aging process for prop versions.
Mr. Kehoe also touches on the plethora of materials used as closures. Beyond the stereotypical cork you'll find leather, cloth, pewter, and, as in this container of Crocus Martis, parchment. All potentially combined with further sealants including, wax, pitch, and tree resin.
I would feel a bit guilty if I didn't add some kind of warning. The article on classic medical containers is phenomenally good, but beware wandering into the rest of the Pirate Surgeon website. If you have any interest in history it's a rabbit hole that could easily consume several hours of your day.