Breed a Chansey or Blissey with Luck Incense in the Day Care at Goldenrod City. Or you can tranfer it from your Pokewalker.
You can Chansey at the Safari Zone, and Routes 13, 14, and 15.
Happiny is found at Amity Meadow in the PokeWalker. But that was an Event that passed, so you cannot do it anymore.