There is ano one central database in SF for rentals but the next best thing would be Craigslist. Even the various property management companies use Craigslist for their rentals. The rental market seems to be tightenting up so I'll check Craigslist often and be ready for when a place comes up that you like.
Rich Bennett, Realtor
http://www.LansingStCondo.com http://www.76-78Prosper.com http://www.115-117States.com
Zephyr Real Estate