Blood stem cells are found in bone marrow. Bone marrow is located especially in the skull and some other parts of the human skeleton (ribs, spine, and pelvic bones). This shouldn't be confused though with spinal marrow, which is a part of the nervous system. 

Worries that a donation of bone marrow could cause injury of the spinal marrow, are unfounded.

Blood stem cells can also be found in very small amounts in blood. Through a treatment with special drugs, blood stem cells can be enriched in normal blood, thus enabling a peripheral donation.

Blood stem cells are also found in the umbilical cord and cord blood.