There are only a few ways to get a Snivy in Pokemon Black:
1.) Obviously, you can choose Snivy as your starter Pokemon in the beginning
2.) You can go to the Global Trade Link (which can be accessed in any Pokemon Center on the 2nd floor) and trade one of your Pokemon for Snivy OR you can trade a pokemon for either a Servine or a Serperior and breed it to get a baby Snivy.
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Snivy evolves at level 16
The starter Pokemon are Snivy, Tepig, and Oshawott.
Snivy evolves into Servine at level 17 and Servine evolves into Serperior at level 36.
So far, in Japan, he has Pikachu, Oshawott, Tepig, Tranquill, Snivy, Scraggy, and Swadloon. In the USA he has Pikachu, Oshawott, Tepig, Snivy, and Pidove.
It would take approximately 5,120 steps in order for a Snivy Egg to hatch.