Here are dirt biking related links to help you in various ways. When it comes to regulations and bike parts, it's here.

California State Parks- OHV Recreation Division gives a list of the state parks that have ohv areas and rules and regulations
The Racers Edge Discount motorcycle parts, accessories and service in southern California
Mission Motorsports They sell new dirt bikes, gear, and parts. Great place. Located in Irvine, CA
CycleGear Sells all kinds of dirt/street biking gear. Many locations, one in Laguna Hills
Cycle City A family owned store with gear and parts for all kinds of bikes. Also changes your dirt tires.
Quietbike.org Info about quieter bikes and wildomar and pinnacles staging areas
Loudsucks.com Info and reviews about quieter exhaust systems, News, Apparrel, and Free Stickers
Dirt Bike Fanatic We have accurate and up to date advice on finding the best dirt bikes and motocross bikes around
GotDunes.com Another awesome place that provides helpful info, pictures, and a forum!

Video Links

Gorman Trails Thanks to Igor for supplying the video. Good video on how Gormans trails are.