i've looked online for antique brass snaps high and low and so far the only ones i've been able to find are these:
http://www.ebay.com/itm/Line-20-Snaps-leathercraft-100-pack-Antique-Brass-tandy?item=300311990424&cmd=ViewItem&_trksid=p5197.m7&_trkparms=algo%3DLVI%26itu%3DUCI%26otn%3D4%26po%3DLVI%26ps%3D63%26clkid%3D6887393158189153864#ht_1658wt_907
i've ordered several hundred of these before and they are pretty good. my only complaint is that the cap (the part you hit with a setting tool) is small and isn't very good for thicker leather.