Well I don't know what's considered a lot of agencies for a city but the Bay Area seems to have several  small agencies. To me it seems the Bay Area far more about the independent provider. Likewise it seems it's both an incall and outcall city. Since the Bay Area and beyond encompasses a large area, providers move around and visit different towns and cities for a fews days. So many will stay at a hotel, or some other incall location, in downtown SF for any given number of days. Likewise if they were staying or are based outside of SF I'm sure most providers are more than willing to travel with an added transportation fee, 2 hour minimum etc.  

Here is a list are most of the local agencies: Bay Area Support Service, XO Referrals, Hush SF, Divine Companions, Fantasy Metro.
link is a thread of some previous agency discussion

I had posted this awhile back but got denied because I didn't follow protocol.  Not sure if you are even around anymore.

depends on what you mean by last minute.  If you are the type that calls up and says "I need a girl in an hour" then you probably won't have much luck with an incall.  But if by last minute you mean you didn't make any plans yet, but are in town for a few days, then you may be able to hook up with someone.
 
SF hobby scene is alive and thriving, despite the low traffic on this board.  I would say SF is a "plan ahead town".  Most indies and agencies want at least a few hours notice.
 
HushSF and Fantasy Metro have good reputations
 
HushSF
www.hushsf.com

FantasyMetro
www.fantasymetro.com

Bay Area support services doesn't advertise as an agency, but their women are top notch.

I've had good experience with XOreferrals but others have not.

www.xoreferrals.co