These are common with many/most/all other seats from that era. A stop at a junkyard will allow you to get some. They often show up in boxes of random fasteners at swap meets, too.
Byron Fettig in North Dakota may have them as he salvages and sorts all the pieces from the cars he pulls apart.

Or, are you looking for new versions? Nicely plated?
I don't recall R/T Specialties having these.