Source for rear trim panel retainer?

Hi all, I have a 68 Dart 2 door hardtop and am missing a white plastic retainer that acts as one of the fastening points for the trim panel under the rear quarter window. Does anyone know where I might find one of these? I included a picture from the FSM but this diagram only shows a Charger. The other attached pictures may help to describe it. Thanks!

FSM.jpg

looks_like_this.jpg

reainter_hole.jpg