Where doooo you get rock smash in heartgold?

You get Rock Smash in HeartGold from the Cycling Road after defeating the Elite Four.

Here's how to get it:

1. Defeat the Elite Four and become the Champion.

2. Return to Goldenrod City and visit the Game Corner.

3. Head to the back of the Game Corner to find a small room with a TM on the ground.

4. Pick up the TM, which is TM26 (Rock Smash).

Now you can teach Rock Smash to any compatible Pokémon and use it to break rocks!