Short answer: Everywhere. But if we elaborate on this answer; stem cells are most numerous in babies in the womb, especially when the baby is still in the form of a small blood clot. Since the usage of the cells in this period brings together with it many moral problems, it is not preferred yet.
Adult humans also have similar stem cells that can turn into all types of specialized cells; but some of them are not easily accessible or the number of easily accessible ones is very small. On the other hand, the numbers of stem cells in relatively easily accessible areas such as bone marrow, olfactory nerves are relatively large. Obviously, stem cells can be taken from these areas only by some surgical procedures. However, only some certain types of cells can be obtained from these stem cells.