SOCAL Air Freqs
In Southern CA, there are a ton of freqs between the airports, TRACON (aka SOCAL), and KZLA ARTCC (aka LA Center)
AirNav for San Diego has 119.6 listed as their West departure/approach frequency
AirNav: KSAN - San Diego International Airport
Although I live near LA and not SD, I have looked through a lot of the Southern California air stuff so I thought I may be able to give some suggestions. Based on your signature line, I won't go into the details about differences and altitudes etc.
Although not specific to Southern California, this thread has some good information and links to resources:
http://forums.radioreference.com/aircraft-monitoring-forum/263318-artcc-frequencies.html
After 119.6 the may be handed off to ARTCC although it's also possible there is another TRACON intermediate frequency which is what happens at LAX. If it's TRACON, based on an old FAA SOCAL manual that I have from somewhere, it looks like it would be within the "San Diego" sector/area as opposed to "Coast" (next North after San Diego) and "Del Rey" which are the next two sectors to the North along the coastline. Unfortunately, I don't have specific freqs. for the San Diego TRACON area, but for the Coast area sector you can try "ONeil" at 132.7 or "Shore" at 124.1; you may also try "Pacific" at 128.1. These are based on older diagrams and since I don't live in these areas I can't confirm that they are still accurate, but they are worth giving a shot as a starting point and the FAA tends not to make a ton of freq. changes.
Once they are handed off to ARTCC, depending on their altitude, they will be handed off to Low or High Sectors. "Low Sectors" in that area should be either 9 (more Southern) or 22. According to the database here:
Los Angeles (ZLA) Air Route Traffic Control Center (ARTCC) Scanner Frequencies and Radio Frequency Reference
"9 Low" should be 128.6 and "22 Low" should be 134.575.
For the "High Sectors" it should be Sector 30 which covers quite a bit of Southern California. From the same database, "30 High" is 119.5.
Also, sometimes private aircraft leaving an airfield such as SAN may be given actual hand off frequencies from the controller since they may not be as familiar with the handoffs as the commercial pilots are and with some time, you may hear these freqs. given over the air.
Hope this helps, Southern California airspace is a little crazy for air radio freqs.