You can begin here.  These are the two best known rescues.

National Brussels Griffon Rescue, Inc. is a 501c3 not-for-profit organization which maintains a national rescue league for Brussels Griffons which have been abandoned, lost, abused or for other reasons must be re-homed.

American Brussels Griffon Rescue Alliance is a small, nation-wide, non-profit organization.  Their goal is to find permanent homes for Griffons and Affenpinschers in need.   Most have spent their lives in commercial breeding facilities.