Where do you get rock smash in pokemon saphire?

You learn Rock Smash in Pokémon Sapphire from the "Contest Spectacular" Contest Coordinator in Slateport City.

Here's how to get it:

1. Beat the Gym Leader in Slateport City: You need to defeat the Water-type Gym Leader, Wattson.

2. Visit the Contest Spectacular: The Contest Spectacular is located in the north-western part of Slateport City.

3. Talk to the Coordinator: Approach the Contest Coordinator and they will teach you Rock Smash.

Note: You need to have the HM06 (Rock Smash) in your bag to learn it. You get this HM from Steven Stone after you defeat the Slateport Gym.