Not sure if you have all already heard the news, but Baker Creek Heirloom Seeds has opened a branch in Petaluma! It's called the "Seed Bank", housed inside the old bank building at 199 Petaluma Blvd. North.
My husband and I checked it out a couple of weeks ago and the prices are reasonable. We're new to gardening, and heirloom seeds seem like a great way to grow our own veggies once we're a bit more experienced :-)