Go to a shop that makes custom signs, like the kind you see at every corner advertising a cheap 3/2 with a pool! They all have tons of scrap, and if you are cute, or ask nicely, they should give it you for free. Mine even cuts it to the right dimensions with thier "slicer" if i have the dimensions handy. Even if they charge you(sometimes they don't have scrap of the size i need) its only like 8 bucks for an 8x4 sheet of 1/4 inch corrugated plastic.
let you fingers do the walking and make a few phone calls.
josh