Sube Sports in Huntington Beach. They sell Sparco, Bell, Stand 21, Arai and OMP.
www.subesports.com
A little farther afield, Simpson has a store in Torrance:
http://www.simpsonraceproducts.com/c.../torrance.aspx
and Impact Racing has a store at Irwindale Speedway.
All of them will have Snell SA and M helmets.
Richard