Your Vet will have some there at the clinic, however its much cheaper and better quality to buy the Human form yourself thru a good Vitamin supply store or Health food store. I order mine online from
iherb. its the cheapest place I've found and the best quality. If you buy a different brand just be sure its FREE of sugar,salt,starch,corn, soy,wheat, yeast and preservatives...than it will be safe for cat use.
http://www.iherb.