Google "Lift the dot fasteners". What you will find is you can only buy them in bags of 100 sets for something like $150 or so. At least that's all I could find a couple of years ago when I wanted some of them!
I just looked and found these:
If you need to punch the leather for them you will need one of these:
Another idea is call El Paso Saddlery or Pacific Canvas and Leather and ask if they will install new snaps in your holsters. They both make military replica holsters that use these snaps.