It is coroplast (like the sign making board realtors use). In that post http://www.quiltingboard.com/t-43871-1.htm she describes where to get it, how to cut it, etc. Only downside is there are 27 pages to that post.

I use comic book boards. They're a different size, yet still acid free. And much, much cheaper. And still looks nice:

These are coroplast boards, you can buy them online or I called local sign shops and purchased a bunch of them from them, brought them home and cut them into the needed sizes, it took a little while, but I work very hard for my $ and need to save money, a 20x18 sheet came out to around $1.35 each

At Home depot they are called Coroplast made by Plaskolite they are a company out of Columbus Ohio.

I bought a 4 x 8 sheet of cloroplast and got 48 of the large boards from it for 12.00.
sass

Thank you very much for the information as well as the recent post for the cost of making 48 boards.