Where do you get a Black Flute in HeartGold?

You can't get a Black Flute in Pokémon HeartGold. The Black Flute is only available in Pokémon SoulSilver.

In SoulSilver, you can obtain the Black Flute in the following way:

1. Complete the game: Beat the Elite Four and become the champion.

2. Visit the Lake of Rage: Go to the Lake of Rage and speak to the old man there.

3. Receive the Black Flute: He will give you the Black Flute as a reward for becoming the champion.

The Black Flute is a key item used to attract the Legendary Pokémon, Ho-Oh, to the top of Mt. Silver.