Santa Anita Firing Range
You could try the Santa Anita Firing Range in Duarte, CA (I think).
It's about 25 or 30 miles west of Ontario, follow Interstate 10 then the 210 freeway. You can probably look them up on a search engine (try 'Google') to get a hold of a phone number and address. Also, call to make sure that their open.
~ Golden Bear