These plastic inserts fit into the slots in the door and hold the nails in the door panel nice and tight. The original plastic inserts are usually loose or broken. I can only find these at my local True Value hard ware store. Made by Midwest Fastener.
If you’re talking about these white plastic clips, they are used on early 1960s Impalas. Every Chevy repro parts vendor sells them. This vendor has a pack of 20 for $10:
.