It's on Route 120. You leave fortree, go through the tall grass where you use surf, and then look aorund for a cave. When you find it, go in and surf ot the other side to get it.
Go to the right of Fortree and you will see a giant lake. Surf down in it, and you will see a small cave area. That is Scorched Slab and it contains TM 11 Sunny Day.