I've recently stumbled upon a skin that I absolutely adore, [Blacksmith Poppy](https://cdn3.artstation.com/p/assets/images/images/001/651/895/large/xi-zhang-blacksmith-poppy.jpg?1450179265). Unfortunately, it's been put into the legacy vault _way_ before I even heard about League of Legends.
Money is (almost) no issue, I just want that skin, but I got no idea how it's possible to get it anymore (apart from dumb luck when opening a crate). Does anyone know? Also, how frequently do legacy skins get available again (because it mentioned that they aren't gone forever, unlike Event skins) and in which way?