Both Lowes and Home Depot do sell 1/4" acrylic in 24x48 inch sheets. The price is pretty reasonable (at least at locations near me) especially if you don't have a local place to get it elsewhere, since shipping kills on larger pieces of material.
However, the stuff sold at Lowes and the Depot are extruded acrylic, not [cell] cast. Extruded acrylic swells, warps, and discolors over time when continuously exposed to water. Unfortunately cell cast is more expensive than extruded, but seriously... after a couple years, you'll have wished you did the job right. Cast also machines nicer.
Be sure to verify wherever you get material from that you are getting cast acrylic. Brands do not matter... plexiglas, lucite, blah blah blah it's all good as long as it's acrylic and not ABS or something like that. US Plastics, TAP Plastics, Piedmont Plastics all sell sheet product. I buy PMG Plexiglas G from Piedmont for my projects, but I'm not sure if Piedmont ships and/or has a local location near you.
they even cut it to your needs...and not extremly expensive either
