Hi Diane (and list),
The "classic" SF sourdough is Boudin. They retard the dough for 
*ages* to get a super sour taste. It's worth a try...and their new 
bread museum/bakery is fun to visit and the *only* thing you should 
bother with on Fisherman's Wharf (which is otherwise a useless tourist trap).
My favorite baugettes come from across the Golden Gate Bridge at 
Bouchon Bakery.
If you're doing a full bakery tour in SF, I also recommend visiting 
Tartine great pastries and one excellent bread which comes out of the 
oven at 4 PM each day and quickly sells out) and Bay Bread.
If you would like other SF bread, dining, tourism advice, feel free 
to contact me.
Allen