How do you get a Cubone in Pokemon Pearl?

Talk to your (red clad) rival's sister in Sandgem town. She'll tell you that there's a swarm of Pokemon on a certain route. You might not get Cubone at first but eventually there should be a swarm of them on route 203.

after you get the national dex go to route 203 and you may see one . if not talk to dawn's sister and she will tell you all the old pokemons location and she'll say where it is.