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

Riot’s Terms of Service strictly prohibit the buying and selling of accounts. If a player buys an account loaded with rare skins, they risk a permanent ban once the transaction is detected.

The desire for skins that are no longer obtainable through Riot—such as the Original Championship Riven or the PAX series—has birthed a massive secondary market. However, "buying" rare skins from third-party websites or individual sellers is fraught with danger.

Riot periodically rotates "Prestige" and "Hextech" skins into the Mythic Shop. While these require Mythic Essence (earned through play or loot boxes), it is the only way to officially "buy" skins that were previously retired.

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

Occasionally, the "Your Shop" personalized discount feature can pull "Legacy" skins (skins no longer in the regular store) from the vault based on your play history.