Edmund Scientific and other optical components suppliers for pre-made glass filters in small sizes. Scientific, industrial, amateur astronomer DIY types - Edmund has a ton of stuff.
Rosco theatrical gels. Need a tiny piece, you might just be able to use the $4.00 swatch book to experiment and to make one of your final filter.
Anything used for photography: 2filter.com - this New England mail order outfit carries lots of inventory for stills and video use, including oddball sizes and types. Owner will talk your ear off.
About guns, I know little, having shot a .22 rifle as a kid at camp. The disadvantage of hunting to me at least is that if you are successful, then there's 100+ pounds of deer to drag back. I thank the diligent hunters out there keeping more white-tails off the local highways. There sure seems to be a lot of sights and other accessories at the local Cabela's/Bass Pro/Gander Mtn (latter two went down the drain or were absorbed by Cabela's, or vice versa, I think). Maybe what you want is just sitting there at MegaMall Outdoorsy store of your choice.