I've gotten most of my molds on eBay, especially the last few years. Probably won't find one "cheap", speshly the newer molds like the screw lock football which would also be tuff to find used. Even the older molds have gotten pricey, just paid $37(including shipping) for an RHB-7-14 which tapered barb version I've been waiting on for years. And the "looks like its never been used" wasn't nearly accurate, in fact one cavity was compromised by a Dremel oopsie.
You'd probably be better off shopping Zeiners, Barlows, or other online suppliers. Just, I like the older molds better.