Here's how to find it:
1. Go to Goldenrod City: This is the city with the Radio Tower and the Goldenrod Department Store.
2. Look for the building with the Pokeball symbol: This building is located in the northern part of the city, near the Gym.
3. Enter the building: Inside, you'll find the Move Tutor.
4. Talk to the Move Tutor: He will offer you a variety of moves to teach your Pokemon.
5. Choose Rock Smash: Rock Smash will be available for a price.
Important Notes:
* You need to have obtained the HM for Rock Smash before the Move Tutor will teach it to your Pokemon. You will receive this HM in the Ilex Forest after defeating the Gym Leader.
* The move cannot be learned by all Pokemon. Only certain Pokemon can learn Rock Smash, so make sure your Pokemon is compatible.
* The cost to teach Rock Smash varies depending on your progress in the game.
Good luck finding Rock Smash!