You can probably grow most any winter cover crop in central California.
Here is a blend from Peaceful Valley:
http://www.groworganic.com/soil-builder-mix-raw-lb.html
Another from LA Hearne:
http://www.hearneseed.com/product-in...ted-pid85.html
Crimson Clover is kind of pricy, but it should do well in your climate
for a winter cover crop.
A guide from UCDavis with fairly wide coverage of cover crop species
used in California:
http://www.sarep.ucdavis.edu/cgi-bin/ccrop.exe