Mmmm, good luck. There "used to be"... dairies, and various small farm lands around, but that's been gone many years now.
You might try your luck at the dairies in the Chino area. I'm sure they have a pet problem, Or maybe the Chino Air Port. But in the general San Gabriel area (Pasadena, Duarte, Covina, Glendale, etc), no police are going to let you shoot out doors.
There are many shooting ranges in a 50mi radius from Pasadena. I'm betting they will let you shoot, but non-breathing targets only !
I'll be going to an indoor range in Upland (Magnum Shooting Range) soon. I'm gonna ask if they'll allow air guns.
Mike