How do you get the Snowdown 2013 - Giftwrapped Poro?
It was a mission or event reward. Riot's description in the client reads: “Earned by winning 3 matchmade games on Howling Abyss during the 2013 Snowdown event.”
Is the Snowdown 2013 - Giftwrapped Poro still available?
No. Riot tied the Snowdown 2013 - Giftwrapped Poro to 2013 (mission reward), and it does not normally bring back past event rewards or bundles.
