Where can I buy a kite in San Francisco?

You might look into the games store on Divis and just south of Oak (or Fell?).

there is a kite shop in chinatown

http://www.chinatownkite.com/

At the Berkeley Marina there is almost always a man with a long white beard flying and selling kites. Sometimes he brings the ones he makes - they are amazing, fantastical, humongous kite creatures that cost upwards of $500 (think work-of-art). But he also sells regular trick kites. If you are a kite lover it's worth it to take a trip there anyway on the next breezy day just to see all of the kites in action, his and other people's.

Soko Hardware (at 1698 Post Street in Japantown) sells some great Japanese-style kites.