Were can you get an aipom with a trainer Pokemon diamond and pearl?

You can't get an Aipom in Pokémon Diamond and Pearl. Aipom was introduced in Generation II (Gold, Silver, Crystal), and Diamond and Pearl are Generation IV games.

However, you can find Aipom in its evolution, Ambipom, in Pokémon Diamond and Pearl:

* Ambipom is available in the Grand Underground, specifically in the Iron Island. It's a rare encounter, but you can find it in the underground caves.

To get Ambipom, you'll need to first catch Aipom in a previous generation game and then transfer it to your Diamond or Pearl game using the Pokémon Bank and Poké Transporter services.