Do you want to volunteer to again experience in a dental setting? Just go to a dental office and ask. Or you want to volunteer at a clinic to not only gain experience but be of actual use to clinic...which they can't afford to pay to have lots of dental assistants...if this, then Santa Ana has several clinics and welcome volunteers as long as you are serious about it and don't flake on them, also San Diego has dental clinic offering services to low income families.
I asked one of the clinics in Santa Ana if I could volunteer there and they said "I'll put you on my interest list and update you via email." That's a no... 🙁