If you're running a WWII era game you'll find a plethora of prop paperwork over at Hardscrabble Farm. The site's collection of military ephemera is intended for historical reenactors, but I can think of a lot of ways they could be incorporated into scenarios. Just looking at the transit passes, signal corp envelopes, and explosives labels (!) brings to mind all kinds of possibilities.
Browse around the site and you'll also find some great WWII and Civil War era prop photographs.