Do they *have* to be glass? If not, you can get some nice Lexan that won't warp from a few different sellers on eBay. Most of those sellers will do custom cuts for you.
If you want glass, I've had good luck with Glass Cages. You can google around for reviews on their tanks - they're not good - but the glass tops they make are pretty nice. I have several of the tops. With shipping, it would probably just be cheaper to buy at your local shops, though.