Where to catch haunter on Pokemon platuim?

In Pokémon Platinum, you can catch Haunter by evolving a Gastly or by finding one in the wild. Gastly can be found in the Old Chateau, specifically in the room with the TV on the second floor. After capturing a Gastly, you can evolve it into Haunter at level 25. Alternatively, you can trade for a Haunter from another player, as it evolves from Gastly through leveling up.