Even though you're by the ocean, i don't think if you caught one in the wild it would survive in a tank. you need to get one that's born in captivity. (We tried it) You'll probably have to go to a fish store that specializes in marine fish, and you should have no problem finding some in San Francisco, I would think.
There are mail order places you can get them from that you can find on the web, but I'd rather take my chances from a local shop. I wouldn't want to take the risk of keeping alive a starfish that has gone through all the stress of being shipped. It will probably have enough stress just being introduced into a new tank without all the added stress to it.
good luck I hope you find one. I subscribe to Aquarium Fish magazine, and there's been pictures of some pretty awesome starfish featured in it.
EDIT: hey shiny, here's a site for you to check out: www.drsfostersmith.com. When you get to the site, click on "live aquaria deliveries on the left site in tan box, then click on "marine inverts" or type in "sea stars" in the search box. I get alot of fish supplies from them, but haven't even ordered anything live. They have some pretty cool looking ones there.