On heart gold how do you get a red stone?

You can't get a Red Stone in Pokémon HeartGold. Red Stones are introduced in Pokémon Ruby, Sapphire, and Emerald, and are not obtainable in the Generation II games.

If you're trying to obtain a specific item for a particular Pokémon, you might be confusing it with another item. Perhaps you were thinking of a different item with similar properties?

Could you please clarify what you're trying to do? I might be able to help you find the correct item or method for obtaining it.