The best deal I found was on Amazon the Lantee "brand" "push fasteners" aka "plastic rivets". 6mm thick and they fit perfect and you get a 100 pack for ~$9? but... the listing is inconsistent and sometimes it appears to get switched out with something very similar but no correct. These fasteners look and fit perfect! I tried so many others that were a little to big or small.
Currently, I found this listing on Amazon (don't trust the link) verify that it is 6mm before you buy.
Description: "Lantee 100 Pcs Car Front Fender Skirt Push-Type Plastic Nylon Retainer Clips Fasteners Rivets 15 x 12 x 6mm"
If only there was a link.
Oh, wait, here it is: https://www.amazon.com/Lantee-Pieces-Fender-Plastic-Fastener/dp/B01LWRX0O9