If you’re looking to grab a key just for Emerald’s unlocking skill, you can find a few pretty early either at Mt. Guido’s Hundred Hole Cave section in the very top left cave with Secret Search and a Crash Hammer (which can be bought at Coriander Maze), or World End Tower, which requires the sailing boat. Some Mimics also drop it if Treasure Eye is used. If you want a more steady supply, Grom starts selling them on the airship if you buy from him about 30 times.