This Pokémon was originally obtained through an event which is no longer available. It was provided through the Mystery Event feature as a Key Item, known as Old Sea Map. Upon displaying this to a ferryman, he would take you to Faraway Island, where you could chase a level 30 Mew around the tall grass, attempting to intercept its path to fight it. There are no known (legal) methods of obtaining Mew in the GBA versions of Pokémon