Most of the ones you see today are screw-backs rather than rivets, but you can still (sometimes) find the old rivet style. Look for "sam Browne studs". Tandy should have them, or a multitude of suppliers on AmaZon and eBay.
(But honestly, the last time I needed one--for a shot pouch--it was quicker to make it than to find a source. Less trying of my patience. . . .)