If you don't find it from someone on the board, you can probably find it in your area. Get your local phone book out and look in the yellow pages under plastic. Any of them that sell sheet may also have scrap, as many of them do fabrication work in addition to selling. Often they sell the scrap by the pound.
You might also check with Precision Marine. They have made some skimmers with colored boxes, and might have some scrap that they can't readily use. If you're lucky, you might be able to purchase some small pieces.
Many PC-Mod stores sells acrylic sheets. Search for "PC Mod" in search engine. They are usually located under "windows kits". However, the largest I know of they typically carries is 16x16, which is the average size for the PC tower side panels.
Try US Plastics. I've ordered big sheets (mine were 96x24, 1/2" thick) from them bofore. The sell sheets up to 96x48, 1" thick. Good prices, reasonable shipping. http://www.usplastic.com