[Quiet Man Cometh]

Umbilical cord blood is a big source, which is something that is normally thrown out anyway so really, nothing is lost (I remember hearing a story where a the stem cells from a baby's umbilical cord were used on the mother to cure her leukemia.) For the purposes of tissue creation, they can use normal cells and sort of grow them. At least, this is what I'm getting from the wiki on it. ;)