Tandy Leather Company has a lot of conchos and buckles, too. Also conchos.com, if you're buying in bulk.
Get the hole-punch, Chicago screw kind. You can buy Swarovski crystals to put on them, very easily. A lot of people use E-6000 glue with good results, but I've always preferred epoxy....unfortunately, I don't know why kind my industrial-grade stuff is, or where to buy it commercially. Buckles can be changed out, too, but depending on how yours are put on you may have to get creative. If they're sewed or riveted on, you'll just have to destroy that and replace the stitching with a Chicago screw.