How do you get the stormy sketch on Pokemon LeafGreen?

You can't get the Stormy Sketch in Pokemon LeafGreen. The Stormy Sketch is a special item introduced in Pokemon Diamond, Pearl, and Platinum. It is a rare item that allows players to learn the move "Weather Ball" with a specific type depending on the current weather conditions.

LeafGreen is a Gen 3 game, and does not include this item or the move "Weather Ball".