I found the lowest-cost plastic sheet.

http://www.flickr.com/photos/100044751@N08/10488521405/
I found the lowest-cost plastic sheet.

bonbrobeck posted a photo:

I found the lowest-cost plastic sheet. Including: ABS Sheet, Sintra and PETG sheet. All of these are thermoforming grades of plastic sheet. If you need for plastic materials that are formable this is the cheapest place I have found. www.dakkadakka.com/dakkaforum/posts/list/229673.page