For ease of travel, the no-brainer answer is to fly to SNA from SFO (or whatever Bay Area airport is closest to your starting point). Be sure to do a separate search for Southwest, as most travel sites do not list them.

For lowest cost, there are plenty of other options that might be cheaper, including buses, trains, driving all the way from SFO, or flying to a different Southern California airport such as LAX. But all of these alternatives trade off dollar cost for time and frustration. If your time and (lack of) frustration mean anything to you, just book the SFO-SNA flight.