Here's how you could get Shaymin in Pokémon Diamond:
* Event Distribution: Shaymin was previously distributed via events. These events are no longer active. You might be able to find someone who still has a Shaymin from an event and can trade it to you.
* Trading: If you have a copy of Pokémon Platinum or a later game where Shaymin is obtainable, you could trade it to your Diamond game.
* Cheating: Using cheats or save editors to obtain Shaymin is considered unethical and against the spirit of the game. It's not recommended to do this.
Important note: Even if you can find someone with an event Shaymin, there's no guarantee that it will be compatible with your version of Pokémon Diamond. The game's generation and region can affect compatibility.