This is the material that the Zagi type of flying wings are generally made of. It looks more or less like eps (expanded polystyrene) - white foam. However, this foam, when compressed, bounces back with some resilience. I could not find any locally (St. Louis), but you can order it on-line. I do not have any web sites right now, but you can probably find them with google, etc.

I cut my flying wing from this stuff using the same homemade foam cutter As eps. Takes a little more heat but otherwise cuts fine.