Great for self mobilization of muscles and improving range of motion of the spine, you get these at Amazon.com. We usually recommend the 36″x6″ white foam roll for general use, but often the shorter 18″x6″ might be adequate if you are using it for your lower extremities.

Foam Roller at Amazon.com
If you are in Manhattan, Paragon (18th & Broadway) sometimes carries them but often limited in stock.