I have an adult mentally retarded child and recently moved to California. We are in the process of placing her with what looks like an excellent day program, but are planning on her living with us until we can no longer care for her.
Their program looks pretty good to us, which is the reason we moved here from the midwest. You can look at information from these sites, they have contact info too:
Alta California Regional Center
A Consumer's Guide to the Lanterman Act (broken link)
Best of luck, you need to spend some time discussing your childs abilities and limits with a social worker to work out the best placement. Their are lots of options, but sometimes you can be on a waiting list for a while.