abs plastic? Have you asked your local car audio shop for a piece? I used to sell it to customers for about 5 bucks a foot. If not, you can order it from most online car audio shops, I found this one for just the size you want:
What thickness do you want? Cintra plastic sheet can be found in various sizes, thicknesses and colors. Also look for Styrene plastic sheet. Depending on the thickness of the material you wish to find, along with the application, will determine the type ofmaterial you would have the best luck with using. Look in the yellow pages under "Plastics", call a few suppliers and you're guaranteed to find exactly what you want, whether is be cintra, styrene or abs. You might even have luck finding this plastic at art supply stores, though they generally sell white (I have seen some stores carry black, however) and typically thin sheet stock. Michael's arts and craft store might carry styrene.
I wished the buck stopped here, as I could use a few.