I have bought foam from this place http://www.foambymail.com/
They have great service. I use 3/4" polyethylene and 1/4" minicell in my pack. The 1/4 stays in the sleeve and the poly goes in before anything gets packed. Both are inside the pack. The poly comes out at camp and makes a great sit pad. I think the poly has lost its firmness and is need of replacement. I could continue to use it, but I have a huge sheet, so why not. It has about 400 trail miles. I tried using election sign material, but it weighed the same as the poly and had no multiuse that I could conceive, Plus the edge of it was sorta sharp and sorta rough no matter how I tried to cut it and I was afraid it would wear a hole in my pack.