Depends on the car and what you need but I have used Partsgeek.com for an alternator and the part was on my door step the next day from UPS because they have a warehouse in socal. For actual body parts (fenders, bumpers...) Levan's they are in Oakland, SJ and Sac, I just bought a bunch of aftermarket body parts for my BMW and they were dirt cheap, they do sell some mechanical parts now but I am not sure which ones.
And pick-n-pull is always cool if they have your car, the one in Oakland isn't that great but the one in Fairfield is cool and the one in Richmond is better than the Oakland one.