There's the Apollo XI field at the Sepulveda Basin in Van Nuys, CA. This field is part of the Los Angeles Parks and Recreations and is open to the public. No fees, club or AMA membership is required. I would recommend checking out the website by the San Fernando Valley RC Flyers for directions and to see if there are any events that may close the field to the general public. The field is in the San Fernando Valley, about 15 or so miles north of downtown LA where the 405 and 101 freeways meet.
http://www.valleyflyers.com/default_flash.htm
Scott