The short of it - Embryonic stem cells come from embryos. Usually at that stage, the baby is referred to as a "blastocyst" and resembles a ball of cells. This is about 4-5 days after fertilization, when normally the baby would be moving down the fallopian tubes into the uterus. To "harvest" cells, a scientist will "scrape" the insides of the blastocyst, or flatten it onto a petri dish. Another technique is called budding - when the baby is only about 8 cells, the scientist will separate one of those cells and use that to grow stem cells. Getting into semantics, this process is still considered morally unacceptable since if budding happened in nature, identical twins would have formed. Also, in the research done to come up with this process, all of the hosts the cell came from were neglected and died. The biggest source of embryos comes from the unused frozen leftovers of IVF. Usually this is between 6-100 embryos, usually less. In order to have a theoretically feasible treatment using embryonic stem cells, about 8 million embryos are needed. Considering woman have fewer egg cells, embryonic stem cell treatment is not feasible. I know I said otherwise earlier, but another source scientists would like to exploit for embryonic stem cells come from aborted female fetuses. The idea is to use all the egg cells in their ovaries for embryos, and it would "solve" the problem of the number of embryos needed.