You can only get it at the beginning of the game unless you go to the gts at the top right of any Pokemon center and trade to get one .
It is one of the three starter Pokemon
When you choose a starter Pokemon if you choose snivy, you might get a female snivy.
You have to get a Snivy from Professor Juniper at the start of your journey then evolve into Servine then into Serperior
oshawott snivy tepig
You don't catch snivy you receive him at the beginning of Pokemon Black or White from professor Juniper.
At level 17, Snivy evolves into Servine.
The only way to find a Snivy is to get it from the starter Pokemon. If not, then you'll have to get it over the Global Trade.
For you to get a life!
tepig oshowatt snivy
The Starter Pokémon in Pokémon Black 2 and Pokémon White 2 are Oshawott, Tepig and Snivy.
Snivy lvl17-> Servine lvl36-> Serperior.
The first Pokemon is Snivy, Tepig. Oshawott.