two other options: go to target and buy the mattress foam(like the hospital foam for elderly people's beds) turn upside down(flat side up) and layer away...also,since you're in chicago, try calumet photo...i think they may sell the foam(but might be a little pricey) good luck

Foam
 

Best place to buy "Pro" Foam would be a boating supply store.

Here is an example...

http://www.defender.com/product.jsp?path=-1|7504|320537|61628|552834&id=61779

You can buy it by the square foot. As much as your project needs. Just type in "Foam" in their search box.

Be sure to upload/post some pictures of your finished project.

-Corey

http://www.independentfilm.com

I got lucky on the 40.00
It was miss marked, normal price was 130.00

I used a large bread knife to cut the foam, and used foam glue to put it all together.