Heart scales are only in Pokemon emerald, ruby and sapphire but you can get one if you trade a heart scale from ruby or emerald or sapphire to leafgreen then take it but it will not do anything in the game.
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you have to use hm rock smash on rocks to find them.
There's a man in Fallabur town who will re-teach a Pokemon a move for a heart scale. But you must find a heart scale, they are in several places on the map but are mostly in or near water.
Heart Scales can be purchased in the Pokeathlon shop for 1000P
Piplup does not exist in Pokemon LeafGreen or FireRed.
You can find the Heart Scale under ground by using the Explorer Kit.