You should be able to get it at any glass shop . It is really expensive though. But what isn't in this hobby !

Home Depot, etc... And yeah, not cheap.
http://www.homedepot.ca/product/poly...44-inch/924860
I bought 4 X 8 sheet at Industrial Plastics some time ago, and can't remember what I paid for it, but didn't think it was that expensive. I use the 1/16" thickness. You can bend a flange to gain rigidity where you need it. Also glues nicely with Weldon. And very good with light transmission, and used by commercial greenhouse.

What I like about it, too, is that it can be bent and shaped just like metal sheet, and does not break. Here is a 7" filter sock hanger I made from scraps for one of my sumps. Worked great.