If you are a local to the Bay area, Clay People in Richmond is a great resource for commercially produced clay.
For handbuilding, use earthenware clay that is strong and easy to shape. Store it in a plastic bag to keep it moist. It is easiest to work with premixed, wet clay. If you are a local to the Bay area, Clay People in Richmond is a great resource for commercially produced clay. You can also get clay online from a vendor like Laguna Clay Company.