When the Master Amulets were released, Professor Greyrose said in a post that the Master Amulets would be dropped in Wintertusk. So far, none have been found. Does this claim hold true, or by "Wintertusk," did you mean the "Waterworks?"
This is pretty much in the same way that you could get a Jade Oni pet from THE Jade Oni: very very very rarely. They're are drops for them out there, but not very common ones. However, I have heard a few people say they got one from the final Waterworks boss. (Both of them got Myth amulets, but again this is simply their word, I don't have anything to validate it.)