There are plenty of places to buy a tray of oysters up near Tomales Bay, and a recent thread debated the experience of dining at two heavy hitters: Hog Island Oyster Co. and Tomales Bay Oyster Company.
According to hummy, it comes down to how well you packed your other picnic supplies:
"My impression was that [Tomales Bay Oyster Company] is good if you bring your own stuff (wine, cheese, bread, cooking supplies, shucking supplies, etc.) and [Hog Island] is good if you don't bring your own stuff since everything you would want is available for sale."
Hog Island Oyster Co. [North Bay]
20215 Highway 1, Marshall
Tomales Bay Oyster Company [North Bay]
15479 Highway 1, Marshall