That HM is not in the game. I'm sorry
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you find defog at the safari zone right when you walk in you go to a girl and she gives you defog Toe311,but its diferrent on platinum
you get hm strength and hm defog after defeating two gym leader.Fantina(defog) Byron(strength).
you go to blackhorn city and there you will find a place there
Defog did not become a move until Generation 6 so you will not be able to find it in LeafGreen
you will find waterfall on the 1st floor on ice path