Where do you find rocket suit in heartgold?

You don't find a rocket suit in Pokémon HeartGold. The Rocket suit is a piece of clothing worn by members of Team Rocket, a criminal organization in the Pokémon games. While Team Rocket is a significant part of the story in HeartGold, you don't obtain their clothing.

Perhaps you're thinking of a different item or a different game? Could you give me more details about what you're looking for?