Hello marsgall,
If you are referring to the vacuum diaphragm that is mounted to the carburetor ,this is called a vacuum brake or choke pull off.You should be able to get this at a good parts house that carries carburetor overhaul kits.
OK,my miss-understanding.See if this helps you.
Master A.S.E Automotive technician
A.S.E MasterTechnician A1-A8 w/L1,38 years experience General Motors Master Technician