Snivy is not available in Pokémon Violet as a normal wild encounter
You can get Snivy by transferring it from Pokémon HOME if you already have one from another game
You can obtain Snivy through trading with another player who has one
You can get Snivy from events or distributions if one is ever made available
If you want to use Snivy in Violet, make sure it is compatible with Pokémon HOME and then move it into your save file