The finest advice comes from Corki!

How can I obtain this skin?

2009 began with the introduction of the Corki skin. It was distributed free of charge to all League of Legends players as  league of legends level 30 account  payment for Riot winning the reader’s choice award for 2009 from Gamespy and IGN. This skin is unlocked if your account was active at the time. You would also receive the champion Corki for free if you did not already have one.

Is the skin rare?

 As was already noted, there has never been a means to get this skin in-game. The only time you had a chance was in the very early stages of the game in 2009. Because you have to be present to obtain it, UFO Corki is one of the most elusive skins in League of Legends.

UFO Corki purchase:

This skin has never been offered for sale, either in-game or online, and unlike other rare skins, it has never been distributed through code. Therefore, the only way for you to obtain the skin is to purchase one of the accounts that we are now offering for sale on Lolskinshop. Many of our really ancient beta accounts are just waiting for a new owner to fly in the heavenly skin. Visit our King Rammus and Ufo Corki sales page for additional information if you’re interested.