How do you find azure flute in Pokemon platinum?

You can't find an Azure Flute in Pokemon Platinum. It's an item from Pokemon FireRed, LeafGreen, and Emerald.

In Pokemon Platinum, you can find Whirlwind Flute instead, which is used to learn Whirlwind move. This flute is located in the Valley Windworks in the Flottom Mine.

If you are looking for the Azure Flute in another Pokemon game, please specify which game you are playing.