The Old Sea Map was only available as an event Key Item exclusively for players with a Japanese copy of Pokémon Emerald, which allowed players to reach the Faraway Island, and ultimately battle the Mythical Mew. For players who rather not use the Mystery Gift to activate the event, simply use PKHeX's Event Flags > Research, and For Pokémon Emerald: Check Flag 0316 and 2262, then proceed to add a Old Sea Map into the Key Items of the Bag. This is the only Mew that can be encountered