I currently live in Southern California, all of the canyons I've done are in the San Gabriels. There are definitely canyoneering resources out there. The one I use most is rope wiki. Here's a example of the most recent canyon we did:
http://ropewiki.com/Eaton_Canyon People have actually died/gotten severely injured in that one, but none of them canyoneers from what I've heard. Safety is always our number one concern. No matter what we do, we never go alone, someone on the outside always knows where we area and when to expect to hear from us, and we always bring enough gear to be able to ascend in case we need to. Therrin is also trained in confined space and rope rescue for work and was a certified EMT back in the day. Plus he has over a decade of rope experience. I probably would have been a little weary of getting into this kind of stuff without him around.
During one grotto meeting we had a speaker who does slot canyons in the Grand Canyon speak and he had done canyons that there were no record of anyone else having done before. To me that is scary. Once you drop your first rappel in a canyon, you're essentially trapped in the canyon. You pull the rope behind you so you can't go back up, the only way is down. If you get to a point where there's nothing to tie an anchor to at a drop, or where you don't have enough rope for the drop, you're pretty much screwed. He had some amazing photos and videos to show for it though.