From Bulbapedia, the community-driven Pokémon encyclopedia.
Final Island (Japanese: さいごのしま Final Island) is a Friend Area in Pokémon Mystery Dungeon: Red Rescue Team and Blue Rescue Team that can be bought from Wigglytuff in Pokémon Square after beating Rayquaza. It costs 8,500 Poké. It is the only Friend Area in the Western Isles area, and has a capacity of only one Pokémon.
Final Island is a crag of mossy rock that appears to be floating in mid-air, which mimics Mew's ability to levitate. Other smaller rocks surround it, and there is a silver, circular pedestal in the middle.
This legendary island may not even exist. It hovers on the horizon and disappears suddenly as if it were a mirage.