I looked into doing the same thing around a year ago.
Lots of molds out there and there are guys here that are willing to lend them to you in return for a decoy or two.
Two routes to go:
EPS Foam Beads: Easy to work with, just coat mold with corn oil, "fill" with beads and boil. Out pops a decoy. Problem is that you have to find a local source of beads or the shipping will absolutely kill you. Why? Cause EPS beads are "hazardous cargo" or some bs like that. Plus boiling them one head & one body at a time can take a while and you can go through a lot of gas in the process.
2 Part Foam: Easy to get, easy to figure out the mixture. Same deal, coat mold with release agent, pour in foam, out pops a decoy. The problem is that now you have to get the release agent off or paint wont stick. You can sand or sand blast the whole decoy or figure out the trade secret release agent removal method other guys who sell them use. But they arent talking and I dont blame them.
I do hear that if you use a hard plastic coat you can use regular talcum powder as a release agent but have no confirmed that. My understanding of this method, is that you coat the mold with release agent, coat the mold with the plastic coat then fill the mold with 2-part foam.
In the end, I carved decoys from old scrap EPS foam logs I found scattered around after the oil spill and then burlapped them.