Few others have mentioned Lone Pine Gardens, which is fantastic, esp. for pre-bonsai material. They have been open throughout the pandemic.
Encinal is great for soil mixes. If you go looking for bonsai material - speak to someone at the desk.
For CA especially, would reach out to local bonsai clubs and read newsletters. Hard to top a club event for material.
I've ordered trees from Evergreen Gardenworks and Mendocino Maples - always pleased with results. Different experience than going in person.