You may certainly wish to use an agent in helping you find a rental property as that individual may be aware of properties that you do not have knowledge of (either because it is on the MLS, or they know of someone wanting to rent, but does not have it advertised.) On your own, you should take a look at the following sites to get a feel for rentals in San Diego: Trulia, SignonSanDiego, Craigslist, Backpage, San Diego Association of Realtors. There is no substitute for doing your due diligence, especially on Craigslist and Backpage, as I believe those sites are more susceptible to scams, unfortunately.
Avenu Realty Group
Real Estate Broker