Unless you have an exemption listed in the law, it’s illegal for the average Californian to purchase ammo out of state and import it to California. The rumor is that the CA DOJ has agents at gun shows and vendors close to the border watching for California customers.
Do the ag stations now ask if you have fruits or vegetables or ammunition?
I have purchased ammo in Reno and Carson City for years, but just before the ammo law went into effect, sporting goods stores and gun shops there began asking for ID to see if you were from California. I doubt most would sell to someone from California now.