I live in Southern California. I did a local search and came up with these. I used "California overland vehicle rental" as my search. Most of the rest of the results look like camper vans. If your not planning on going anywhere rough, Sprinter vans work very well out here. I have one in my off road group. What areas are you planning on visit? I have gps files of the popular areas out here as well as some hidden spots.
EDIT: just noticed you found cypress and was looking for LA/SD. The bottom two are those. My search results had quite a number of businesses. Most of which were LA/SD.
Cypress Overland
San Francisco Bay Overland
Topoterra
Funki adventures
Cypress is sold out now (my fault, I can still not confirm exact dates).
I plan to do one week Joshua Tree and the area east of LA/SD.
But I'm coming from North, Lake Tahoe maybe, via Mono Lake (normal rental car). Flying back from San Diego. One way rental would be superb, but the fees may be too high.
Sprinter is not what I am looking for, but I may end up with...
Thanks for the links. Top terra I had missed, good selection, but pricy .