Joan Balter: Not only is she incredibly helpful and reasonable in terms of cost for repair, she’s one of only two places I’d trust to fix my performance instruments. She’s often out of town, but worth the wait if she is. Joan Balter (510) 524-4333 (note that this is her home+studio, so don’t call too early)!
Ifshin Violins: A larger store that is more “business” oriented, they’re walk-in friendly. Good for repair, buying instruments (they also have a trade in policy), and estimates. http://www.ifshinviolins.com/
If you’re a beginner or advanced beginner looking for an instrument, see this page, too.