Stem cells exist in varying forms in numerous places throughout the body, although with a tendency to decline in number and quality throughout our life cycle. Adult stem cells are most abundant in bone marrow, fat tissue, and blood; it is from these sources the body often recruits stem cells when they are needed. Bone marrow transplant has been a routine treatment for decades.
Stem cells are especially abundant at birth, in umbilical cord and placenta, which is why many parents choose to privately store umbilical cord blood/tissue when their babies are born. Membranes from placenta have been used to treat burns and other wounds for over a century.