The game doesn't offer any way to obtain this coveted item. This is a common feature in the Pokemon series, where the Master Ball is a rare and powerful item that is usually given to you early on in the game or obtained through specific events.
It's important to note that while you can't get a Master Ball in Pokemon XD, there are still some very effective ways to catch Pokemon, including using other types of Poke Balls and using strategies like weakening the Pokemon before attempting to catch it.