OK Syd, Hikari will tell you:
You can trade for Cottonee (White 2) or Petilil (Black 2) on Route 4 if you have the opposite one of the Grass-type duo. So, that's not too far into the game.
You will need a Cottonee to trade for a Petilil (B2) or a Petilil to trade for Cottonee (W2). Hope that helps!