Update : I did some research on the skins for this account, here are the SPECIAL Skins that can't be bought at the moment.
Judgement Kayle - Limited Edition: 10 matched games in Season 1
Victorious Janna - Limited Edition: Gold or Platinum medal in Season 2
Haunting Nocturne - Limited Edition : Harrowing 2011
Leopard Nidalee - Legacy
Noxus Poppy - Legacy
Traditional Trundle, Karma and Sejuani skins