The two big ones are still in town: Rural/Metro and AMR. Both will start you out doing interfacility transports with RM eventually allowing you to work 911. Uh, stay away from the BLS companies, shady shady shady... shady. Work for E1R, standby event stuff, legit.
I just applied for the EMT-B Position that Rural/Metro has open for Pacific Bowers Ambulance. Also, AMR is currently hiring EMT's. I'm looking for a job myself. I'm fresh out of EMT school so I'm kinda looking for anyone to take me. Here's a list of all the ambulance services in San Diego with their websites. Let me know if you find anything. Good luck!