Articuno is a legendary Pokémon from Generation 1, and it's not available in Black 2 (or White 2). The only way to get Articuno in those games is through trading with someone who has it from a previous generation game.
However, you can find other legendary Pokémon in Black 2, such as:
* Reshiram (Black version only)
* Zekrom (White version only)
* Landorus, Tornadus, Thundurus
* Cobalion, Terrakion, Virizion
* Keldeo
If you're looking for Articuno, you'll have to look for it in other games!