How do you get swablu in pokemon diamond?

You can't get Swablu directly in Pokémon Diamond. It evolves from Wailmer which you can find in the following locations:

* Route 219: This is the easiest and most reliable place to find Wailmer. It's a common encounter in the water here.

* Lake Valor: You can encounter Wailmer in the lake itself, but it's less common than on Route 219.

To evolve Wailmer into Swablu, you need to trade it to Pokémon Pearl or Platinum. It will evolve into Swablu at level 40 in those games.

Unfortunately, Swablu isn't available in Diamond unless you trade with someone who has it from Pearl or Platinum.