Wal*Mart has a new line in the modeling section called "cast and paint"
It has at least 3 sets: Dino bones, cars and the human hand.
The bones and cars let you use plaster in pre-made molds and then paint them. But the coolest looking set lets you make a mold of your hand and then make a plaster copy.
Could this be used for customizing? It seems like it wouldn't be hard to make a two part mold of a head using the "hand" boxed set and then make plaster heads for customs.