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I have found both USPlastics and TAP plastic have good selections of most things thank you.
The only thing i havent been able to find is UV reactive acrylic. I have seen acrylic that in the light looks almost completely clear, but under a blacklight turns blue...
There are many places I deal with as far as acrylic goes, being that my other hobby is reef aquariums and I tend to DIY all sorts of smaller tanks, sumps, reactors, filters, etc.
You can go with larger places like TAP and US, but I find smaller shops nicer to deal with. At that, these places might not have as much variety of materials, as most everything in 'reefing' is black, blue, or clear...but they can get it easily. Many of these places have CNC controlled drills and routers, and sometimes access to laser cutters.
Make no mistake, they might have pre-made equipment and look like big operations, but most of these guys are 1-3 man operations where there are just a few very skilled guys. They will all do DIY/custom stuff, and for rather cheap.
Thats how Im making an external res for my passive water cooling rig...if you look at a 'calcium reactor' at any of the sites above...just imagine a 6' tall acrylic pipe with a locking flange at the top for a res...oooooooo.
Don't forget about local places. If you live in a decent sized city, you should be able to find a place that will sell you scraps for dirt cheap. You can usually get piecs big enough to do just about anything in a case, and depending on what they do they will have lots of differnt colors. I've even got the clear UV reactive stuff. Just be friendly and if you go there often enough they will save the extra good stuff for you.