The Grand Lake farmer's market is only a few freeway exits away from me, but it's such a hassle to park there that I don't go very often. Lucky friends who can walk to it on Saturdays!
Sometimes I have walked the four miles there, shopped, and then carried my groceries home on the bus. AC Transit 57 and N will get you back to our neighborhood.
You could take the N to downtown Oakland, get out at Broadway, and walk eleven blocks - the Friday Old Oakland farmer's market has organic foods, Asian produce, bamboo plants, a chicken roasting truck and tamales stand. And before you load up on goodies to take home, walk down the street to the Friends of the Public Library store and get a used book to read over lunch! The 14 bus will get you to 12th and Broadway, leaving from the heart of our neighborhood.
The 54 bus goes to Fruitvale BART, where the Thursday afternoon market has a few stalls; Sunday has been looking pretty sparse these days, but it may get better in the summer. I like to go to the area for the Chavez library branch, tacos, and hand made ice cream in the Fruitvale Public Market. More on this in a separate post.
The 18 bus goes up Park Boulevard to Montclair - I wouldn't call it convenient for a Sunday farmer's market visit, but if you live near the 18, why not? 20 minutes from Park Blvd. and MacArthur up to Medau Place. You won't have to find parking!