There are lots to choose from, and tastes vary. Here's my favorite: Boudin's (pronounced "bo-deen's" according to their logo).
Unfortunately, it is sold only in San Francisco, as far as I can tell. I've never seen it in Napa or Sonoma or Silicon Valleys. (Oops. A recent search of the Web reveals that they now have bakeries in the Stanford Shopping Center and San Diego, at least. Search for "Boudin bakery" and see what you get.)
As you leave San Francisco to drive up to Napa Valley, stop at their bakery on Fisherman's Wharf and buy several loaves to take with you. (If you get it hot and fragrant, the first loaf probably won't survive beyond Sausalito.)
They can even FedEx a few loaves to you for a not-outrageous price.
All contents copyright (C) 1995,2001, Richard Landau. All rights reserved.