Where to get gardevoir for pokedex in platinum?

You can't get a Gardevoir in Pokémon Platinum directly. Gardevoir is the evolved form of Kirlia, which in turn is the evolved form of Ralts.

Here's how to obtain Gardevoir:

1. Find Ralts: Ralts can be found in the Grand Underground, specifically in the Rocky Cave area. They are relatively rare, so you might have to spend some time searching.

2. Evolve Ralts to Kirlia: Ralts evolves into Kirlia at level 20.

3. Evolve Kirlia to Gardevoir: Kirlia evolves into Gardevoir at level 30.

Important Note: Gardevoir is a Psychic-type Pokémon. To evolve Kirlia into Gardevoir, make sure you have a female Kirlia. Male Kirlia will evolve into Gallade.