This website seems to think they're called "plastic snap closures"
Page on http://www.aubreyrene.com
You can buy large lots of "snap closure baseball caps" here:
Baseball Cap Snap Closure-Baseball Cap Snap Closure Manufacturers, Suppliers and Exporters on Alibaba.comHair Extension
I suspect the actual plastic parts are just injection molded plastic. Nothing really special about those.
If you really want to see the part, I'd suggest you buy a cheap trucker hat and tear it down.