How do you get a metal coat in Pokemon XD?

You can't get a Metal Coat in Pokemon XD: Gale of Darkness. It's a held item that appears in later Pokemon games, specifically in Pokemon Diamond, Pearl, and Platinum.

In these games, the Metal Coat is used to evolve a Metang into a Metagross.

Unfortunately, you'll need to look elsewhere for ways to evolve your Metang in Pokemon XD!