You don't really need to search for anything. The idea is simple:
fence pigs into the
pond, keep them there for a few weeks, feeding them and letting them wallow. It will seal the pond. But In my opinion this is extraneous advice for a dam construction. If you are building ponds on a slope and the dam is necessary to hold in the water (as opposed to excavating a pit on flat
land) then I wouldn't trust the dam to some pigs. Also, not everyone wants to take on the chore of caring for pigs, or at least not in the pond site location.. so yeah, I am curious for advice on this as well. I wish there were more resources on DIY pond construction in situations where a dam is necessary.