The largest unfancy farmer's market in the city, with at least a dozen different farm stands selling fresh produce, some organic, some not. You can find everything here. There are also a handful of prepared food trucks at one end. This location is literally city center, so be prepared for limited parking options and good public transit/walkability. Not really a good neighborhood but should be fine in the morning, and right next to the beautiful main SF public library and Civic Center.
Cons: Limited organic