Without question, ARC. Used to be Association for Retarded Citizens but it's now more politically correct. But they are definitely the ones to go to.
ADAPT might also be helpful though they're I think more about disability rights. If there are any CILs in your area they might be able to help too.
Also, every state has a designated Protection and Advocacy agency. If you google "advocacy (insert state)" you'll more than likely come up with a great list of resources.