If you need something more than a "garden center" and you live in the East Bay you have a few pretty good options including Grand Ave. Ace, the Longs (yeah, the drugstore. They have a freakin awesome nursery.) over by Rockridge, and, my favorite, Berkeley Horticulture.
Ok, their prices are little high so Longs will serve you best when looking for garden standards and annuals. But if you need something special, you have a problem area you need to plant, you want great tomatoes, you want bay natives...you get the idea...B.H. is the way to go. Not only is the staff incredibly knowledgable, they are helpful and encouraging and never condescending if you have questions or are a novice gardener. They offer excellent advice when you come in saying things like "I need something that looks gorgeous but will grow in waterlogged soil that sits in full sun and cant be killed by the dog." I look for any excuse to go to this nursery and highly recommend it.