I am not sure if this has been brought up before for those of us in the US, but I found "Cellfoam 88" by Midwest Products Co. at a Blick Art Materials store. The foam has a smooth flat surface, comes in at least two thicknesses 1/4 (I think 4 sheet pack) and 3/8 inches (2 sheets for $4.76), and 11 1/2 inches square. This material eleviates the need to strip the paper from foamcore. The sheet is easy to mark up. Midwest is also a supplier of scale balsa wood.