How To Get A Dratini In Pokemon Pearl?

I think there's a small chance of it appearing after you get the national dex. If you talk to the little sister of Prof. Rowan's assistant in one of the houses in the town of Prof. Rowan's lab (sorry if that's confusing) she says a Pokemon is on a route that is usually there. Basically, go to the town right after Twinleaf, and talk to a little girl in one of the houses. She might say a Dratini is on one of the routes.