Introduced in Pokémon Gold & Silver, Swarm Pokémon make a return in Heart Gold & Soul Silver. In order to find them in this game, you will have to check Professor Oak's Pokémon Talk radio station on the PokéGear. Each day, he will say of Pokémon that have been spotted in a specific area across Johto or Kanto. This changes each day so it's always best to check the radio to see if the Pokémon is available.
The PokéGear and map are also marked with an exclamation mark for the area when a swarm exists.