You need the game My Pokemon Ranch.
>When 999 Pokémon are present, the ranch will expand and jump to level 25, the maximum level. Once this level is obtained, a new wanted Pokémon will be added. Hayley is now asking for any Pokémon Egg. When the egg is brought, she will offer to trade it for her Mew. -Bulbepedia
Once you get the Mew, trade it to your Diamond game.