Re-Cyclery Thrift Shop, operated as a non-profit store by Trips For Kids Marin, is an excellent resource where you'll find all sorts of bicycles and parts, as well as riding clothes and shoes. The store has newsletters and literature about Trips For Kids, so you can learn about what your money is going to support.
Since it is a thrift store, you'll get good deals on all the new and used equipment. For more information, please visit their website or stop by the shop.