Are you referring to what is known as patternmaker's foam? Like Rennshape and other brands?
That is great stuff and I love it. However, the only sources I know of want to sell it only in four foot by eight foot sheets and it costs a fortune! If someone would sell it in smaller quantities, say 1 square foot sheets of the one inch and two inch sheets, I'd sure buy some. It is even easier to carve than balsa or basswood, and fills as easily for painting.