I work in P&C insurance, although not specifically in personal lines or CA.
Generally speaking, you shouldn't have a problem with that experience letter. Your buyers might run into trouble if they get a weird agent or something, but it is pretty standard.
However, State Farm is not an "independent" insurance company. "Independent" insurance agents work with multiple carriers, and because of that can find coverage for more hard-to-place risks. If you talked to, say, an Allstate or Farmers agent, they're going to tell you "sorry, nothing I can do, [Allstate/Farmers] isn't writing new in CA." BUT if you talk to a local independent agent, they might say "Yeah, some of our carriers have pulled out, but we still have available markets" because they work with many different insurance carriers.
My suggestion would be to google "independent insurance agency [city]", and then look at some of their websites to see if they list multiple carriers, like Chubb, CNA, Hartford, Liberty Mutual, Progressive, Travelers, etc., and make some calls.