My daughter sent for this catalog for my truck.I have a 1967 Chevy.The same vents that you have.Here is the catalog to send for "1960-1972 Chevrolet-GMC Truck.
http://www.truckandcarshop.com/
On page 51 is the pr of fresh air vent valve seals at $7.00 pr.67-72.Part #67-05313.