How do you get the Season 1 Silver?
It was a ranked reward. Riot's description in the client reads: “Earned by achieving Silver in the 2011 ranked season.”
Is the Season 1 Silver still available?
No. The Season 1 Silver was a ranked reward: to get it you had to reach Silver during Season 1 (2011). Riot does not give out past ranked rewards again, so it cannot be earned anymore.
