From Consumer Wiki
Stun guns are weapons used to temporarily immobilize a person by the infliction of an electrical charge with no long-term effects. Stun guns may be purchased wherever guns are sold. Minimum age requirements limit sale to individuals 16 years of age who have the written permission of their parents to possess the weapon. However, anyone may purchase the weapon except individuals who use narcotics, who have been convicted of a felony crime involving assault, or a conviction related to the misuse of a stun gun.
California Department of Justice
Office of the Attorney General
1300 I Street
Sacramento, CA 95814
Consumers should check with local city police or county sheriff for local requirements.