Where to purchase depends on what country you're in. I'm from Canada and use the iherb.ca website. They have a good brand called Jarrows Formula that has the d-Biotin.
For longer/faster hair growth I took d-Biotin, about 1000 mg per day. It has to be specifically d-Biotin, not just "biotin." Within two weeks I was cutting my nails so much I stopped taking it!