Is there a way to get Legacy skins when they are not currently on the store?

Ok so before I say anything I am pretty new to league and can't find much info on this subject, let alone recent info. So, I am coming here to ask if there is any way to get legacy skins when they are not in the League of Legends store currently? Some really old posts have talked about "Skin codes" but I don't know if that is recent information and if they still work. If they do work, are there any websites that are trustworthy to sell skin codes? I have noticed so far that I enjoy playing support, and assassins (Mid & Jungle.) I have recently started playing LeBlanc and I am looking for a skin for her, but I definitely do NOT like any of the skins that are currently on the market, but like all of the legacy skins. Thanks in advance