Howdy, Badger, and welcome to Airforums.com. Congratulations on your new-to-you acquisition!
As for the part you need, if it's what I think it is, have a peek in the Commercial forum
; there are several companies listed there that sell the part you need.
There are two sliders on the screen door of my '74: one is the little sliding door that keeps the bugs out, made of plastic. The other one is a slider that attaches the screen door to the solid outer door.
Should we guess which of those it is, or am I not even warm?