Where can I get a Snivy in Pokemon X?

The reason you can only get Snivy in Pokemon X via trading is because it cannot be acquired from Pokemon X (or Y).

Snivy can be obtained in Omega Ruby and Alpha Sapphire from Professor Birch on route 101 after completing the Delta Episode.

Snivy can also be obtained in Black and White (and Black 2 and White 2) as a starter Pokemon.

After obtaining it in one of these games you can trade it between games to Pokemon X.

If you have one of these games you could trade it with yourself or perhaps ask a friend who has this game. Alternatively you may look into asking other people on some forums or something for their assistance in getting you a Snivy.

Snivy cannot be obtained within Pokémon X itself; it can only be acquired directly in Black, White, Black 2, White 2, Omega Ruby, and Alpha Sapphire. You must therefore trade with another player (not an in-game character) who already has a Snivy.