Try “vacation rentals by owners” … I believe the website is http://www.vrbo.com – a great site for short term rentals (1 week to one year) worldwide with numerous offerings in S. Calf and San Diego.

Although most are listed as weekly rentals the owners will “discuss” prices and typically reduce for longer month to month stays. You can also see how much time is actually booked for each property and offer competitively priced bookings if the property has empty periods.