Here's the website for Southern California RID:
Southern California RID
They're a good organization that provides many awesome interpreting workshops, and I'm sure if you contact them they can provide a list of ITPs in your area. I know San Diego Mesa College has one, but I have no idea about its quality, or if that's even close to you.
Etoile is right: interpreting involves a lot more than signing. I've known wonderful signers who were terrible interpreters. Especially since degrees are more and more being required for certification, I highly recommend getting at least an associate's degree in interpreting.