I know nothing about collecting in Southern California, but I know two things from my research, but mostly from talking to collectors.
1) Ain’t nobody giving location suggestions. Doubly so online. Period.
2) The permits are all specific to the land’s administration. National forests have particular rules, but the individual forest will have individual rules. Publicly owned land is good if it’s permitted for the species you want and whoever is managing applications is working with you. Privately owned land can be easier, especially if you’re grabbing an “ugly” landscape tree or offer some money.
In short, starting with permits and working to location is backwards; find out the location (which includes species, aftercare techniques, etc).
Shoot, you’re in SoCal: go to a club, and offer to trade sweat labor to learn all this from someone in the collector who is a known collector.