In most cases in California, a California resident can only purchase handguns listed on the California DOJ's Roster of Certified Handguns.
As of January 1, 2001, no handgun may be manufactured within California, imported into California for sale, lent, given, kept for sale, or offered/exposed for sale unless that handgun model has passed firing, safety, and drop tests and is certified for sale in California by the Department of Justice.
Private party transfers, curio/relic handguns, certain single-action revolvers, and pawn/consignment returns are exempt from the roster requirements.
Visit https://oag.ca.gov/firearms/certified-handguns/search to see the complete list of currently "on roster" handguns in California.
Who is exempt?
CA PEN 32000 provides a comprehensive list of exemptions from the Roster of Certified handguns.
Purchasing an "off roster" firearm
Unless specifically exempt by CA PEN 32000, there are only two ways where a non-exempt individual can purchase an "off roster" firearm.
Purchase the firearm from a private party.