Charlie,
These folks are in Brooklyn Center, MN. I don't know what they charge for shipping but I bought my foam from them and they were the least expensive I could find:
Rochford Supply. Foam comes in 24" x 72" blocks and you cut the foam with an electric carving knife or foam cutter and spray the two pieces with foam glue, push the glued edges together, and it is a new piece to your final size.
Good luck with the cushions. BTW, if you need material, they have a full line of very good material for cheap.