Where do you get the pokeradar?

You get the Poké Radar in Pokémon Diamond, Pearl, and Platinum after you beat the Elite Four and become Champion.

It's given to you by Professor Rowan in his lab in Sandgem Town.

The Poké Radar is a key item for finding Pokémon in the wild, especially rare Pokémon. It's used to find Pokémon that have a similar type or evolution line to the Pokémon you're currently holding in your first party slot.

Let me know if you have any other questions!