The HD, Lowes, and Ace stores in my area only carry thin glass for picture frames and such. I have a friend that works at henderson, so I get pretty good prices there. Lately I have been getting some cheap glass off craigslist and cutting it myself. Its really not as hard as people think. Buy a decent self oiling cutter (around $30) and give it a try. The cutter will probably pay for itself after the first lid. I just cut the glass for a 90 vert from 2 free tanks I got out of the garbage.
Mike