Yep, like the others mentionned, you'll have to find a glass store/distributor.
When I was modifying an enclosure last year, I needed some pieces of glass and looked at all the hardware stores. I finally found a company that specialized in glass and got what I needed but at a ridiculous price. Had I known prior I would have shopped around more.
Keeper of 5 snakes, leopard geckos, 1 green iguana, 20+ tarantulas, 2 dogs & a bunch of rats!