The only way to do so is trade from other 5th generation games. Those being Black, White, Black 2, and White 2.
Source: Experience
You can only get either Snivy, Tepig or Oshawott in Pokémon Black and White. This being at the Start of the Game.
Starters Introduced in the Kanto, Johto, Hoenn or Sinnoh Regions, you can only get by trading for.
Hope I helped :)
