There are several California native oaks and most can make great bonsai. I have several, but should say I have not collected any yet. Traditionally, collecting is best done just before an aggressive growth season. For most trees that means very early spring. In parts of California that can also mean end of summer before a fall growing spurt. If I were in your circumstance I'd try collecting a couple in October, and then again in Jan/Feb. Get as many fine, feeder roots as you can manage. Put in a free draining soil in a container just big enough to contain the roots you've managed to collect. Keep moist but not sloppy wet. I hear that California live oaks will often drop their leaves after collection, so don't give up on them too soon. Join the San Diego Bonsai Club. I'd bet there's someone there with oak collecting experience who would love to swap experience for a collecting site.
Good luck. Let us know how it turns out.