You will probably pick up what you need in Tiburon on your way to Belvedere. At the free way exit is a large shopping center and there are smaller areas to pich things up before you cross the water to get to Belvedere, which is an island off the Tiburon peninsula.
The Boardwalk Shoping Center is in Belvedere (owned by the Belvedere Land Company) Not the greatest shopping, we used to have a pharmacy and a dime store, not any more. Good news on the horizon is that Woodland's Market from Kentfield wiill take over the Boardwalk Market. There is an overlap between Tiburon and Belvedere (a street divides the communities), my belief is that the comminity will support a good supermarket. Most ammenities are about a 10 minute drive away closer to the freeway.
While Belvedere itself has no shopping area of any kind, (being that it is exclusively residential), it is right next to downtown Tiburon, with its charming waterfront shopping area. One can easily walk to the library or the market.