Stryofoam can be a bit messy to work with, and having a vacuum cleaner on hand might help keep things from getting too out of control.
I've used sheet expanded polystyrene, (the type made from the little balls), and also a couple other types as well. There is stuff that is more like filter foam in structure, that you can find in floral stores, but it's fairly expensive. The cheapest is probably found in home centers for house insulation, I've seen fine grained yellow and blue foams, as well as the typical white made from little balls.
When I've done mine, I just went with epoxy mixed up with some sand. Pretty easy, gives a texture/color similar to sandstone, and it's ready to go in a day or two.