How do you get the SRU - Stag?
It was a mission or event reward. Riot's description in the client reads: “Earned by winning a game of Summoner's Rift on Blue side during the 2014 Summoner's Rift Update.”
Is the SRU - Stag still available?
No. Riot tied the SRU - Stag to 2014 (mission reward), and it does not normally bring back past event rewards or bundles.
