Almost any skin in the game (excluding a few ultra-limited ones like Victorious or PAX skins) can technically be "rolled" by combining three skin shards. This is a game of pure luck and deep pockets. The Dark Side: Third-Party Markets

For most players, the safest and most legitimate way to acquire rare skins is through Riot’s internal systems:

In a game with over 1,600 skins, rarity creates a social hierarchy. Most rare skins are "Limited," meaning they were tied to a specific event and never intended to return to the store. Owning a skin like Black Alistar or Young Ryze signals that a player was there at the very beginning—specifically, the 2009 pre-order period. This "prestige by association" makes rare skins highly desirable for collectors who want their profile to stand out in a sea of common cosmetics. Official Paths to Rarity

Are you trying to find the to get them through the game client?