Santa Rosa fruit stands?
Two that are pretty close together and just off 101 where it's easy for you to take Mark West Springs Rd. to Calistoga -
Tierra Vegetables -
Mike's Truck Garden in Fulton -
Edited to add:
Here's a link to the Sonoma County Farm Trails site, where you can search for farms along your route.
Also, in Fulton, you might want to visit Kendall-Jackson's gardens. You can taste whatever is in season usually. No matter what you think of the wines, these folks grow some great tomatoes.