Where to catch Cynthia Pokemon?

Answer 1

You cannot catch the actual Pokémon that Cynthia uses due to the technology of the Poké Balls refusing to work if it detects a trained Pokémon however you can find Pokémon in the wild that Cynthia has in her party.

In Pokémon Diamond and Pearl, Cynthia's party consists of Spiritomb, Garchomp, Gastrodon, Milotic, Roserade and Lucario.

In Diamond and Pearl, Spiritomb can be found once you interact with the Hallowed Tower while you have the Odd Keystone in your possession and have interacted with 32 people in the Sinnoh Underground however you do not have to interact with 32 different people in the Underground.

You can get Garchomp by finding a Gible in Wayward Cave and then evolving it to Gabite at Level 24 and then Gabite can evolve into Garchomp at Level 48.

You can get Gastrodon by finding one to catch in Routes 213, 218, 221, 224, 224 and 230 as well as in the Fuego Ironworks.

You can get Milotic by first using a Super Rod in order to fish for Feebas which is its pre-evolution. Once you have found and caught a Feebas, you will need to raise its Beauty stat up to as high a level as possible and then level up Feebas once in order to evolve it.

For Roserade, you can catch a Roselia by finding wild ones on Routes 212, 221, 224, 225 and 229 as well as in the Trophy Garden and in the Safari Zone. You can use a Shiny Stone in order to evolve it into Roserade.

For Lucario, you will get a Riolu Egg from Riley over by Iron Island. Riolu evolves during the day time as long as its maximum is high and you level it up once.