Have you done a search on Google for "Closed cell foam"?

Ronk,

Two of the major manufacturers of closed cell foam are Fomex and Crest. Are you seeking sheet material or piece parts? If you are seeking finished size materials, try Advanced Materials Corp.

Alchemist

Curiosity got the better of me. What are you making Ronk?

I am looking to make cushions and foot pads of varying thicknesses for use as posing aids.

You didn't say what thickness you are looking for and at first I thought of the 4'x8' sheet insulation foam sold by Home Depot and others, but then I remembered a friend showing me a thick sheet of foam he bummed from building contractors who were using such blocks for a facade of a new building they were putting up. I remember it was at least 4 to 5 inches thick, so maybe that would be more in line of what you're looking for. He and I built and flew radio controlled model airplanes and it is also used a lot in the construction of these planes as it is light, strong, and can be worked with normal hand tools. You might try a few queries with local building contractors and building suppliers in your area. I remember seeing ads for blocks of such foam in model magazines, so if I can dig up a source I'll shoot it on to you. Good luck in the meantime.