When in the U.S. I bought a bunch from Sloan's Wood shop.
http://www.sloanswoodshop.com/acrylic%20sheets.htm
Makes for expensive shipping, I know. I see their thicker red transparent acrylic is sold out, but they still have the thinner stuff, which is easier to work with anyway. Cutting it is a pain, but if you have any shop tools you can do it.
I made my own crude laptop screen with this stuff and some thick foam-core poster-board and ever-useful duct tape. Works great.
Scott