I would also check ebay http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200514437342&ssPageName=STRK:MEWAX:IT heres a start
this auction is for 1/4 sheet
this seller has a lot of 1/4 and 1/8 buy it now auctions you should find soemthing your looking for in the price your looking for.
I just picked up a 30x42 inch sheet of 5/8ths for another project but was watchig this one among others.
You might try Laird Plastics in Enfield. I buy my acrylic sheet there but I know they carry other types. Ask if they have scrap pieces. They just moved there from Springfield, so they are still getting set up. They have been very helpful to me. They usually have scrap room where they sell small pieces by the pound ($1.50).
Laird Plastics (Formerly Modern Plastics)
135 Freshwater Blvd
Enfield, CT
Can't find there phone number but they have an 800 number.