This list is absolutely, by no means, an authoritative list of all FFLs willing to transfer off-list rifles in California. FFLs will only be listed here if it is with their express consent, and to that end, I am calling the ones I know. Many FFLs do not necessarily wish to make it public knowledge that they transfer lowers, and it would be uncool to just list them without their prior consent.
I hope to update this list soon with a number of FFL transfer dealers throughout California.