Its impossible to find a Snivy in the Unova Region. You would have to pick him at the beginning of the game in order to get him. In order to get a Snivy you would have to:
When you begin the game, pick the pokemon on the far left.
Do a global trade in the Pokemon Center and Trade with someone who wants another pokemon you have.
Find someone to who already has a snivy so they can breed one for you. And by breed I mean the daycare. I got an Igglybuff when I put my Ditto and Jigglypuff in there. Gonna try it with my Reshiram XD.....