Here are some sites that should help you out. They feature many GSDs in need of homes in CA.

Petfinder
San Francisco Bay Area GSD Rescue
GSD Rescue of Northern California
GSD Rescue of LA and OC
San Diego GSD Rescue
GSD Rescue
GSD Rescue of Sacramento Valley

Breeders with adult dogs available

Vom Blossm-Land German Shepherds
Nadelhaus German Shepherds
Lundborg-Land Kennels
Lane's House of Shepherds

Good luck!