In Pokemon HeartGold where do you get the masterball?

You don't get a Master Ball in Pokémon HeartGold.

The Master Ball is only available in Pokémon Gold and Silver, where you can find it in the Ilex Forest hidden within a tree.

Unfortunately, it seems like Game Freak decided to remove the Master Ball from HeartGold and SoulSilver, possibly to make the games more challenging.