Misdreavus is one of the few Pokemon Shining Pearl-exclusive Pokemon. This type of ghost can typically be found floating in areas at night. This elusive Pokémon can be a little hard to find if you don't know where to look. Here's where you can find it.
If you are looking to catch Misdreavus, you can start your quest by going to the Eevopolian Forest. You will meet this area soon and have a partner with you during all of this. During this time, you will encounter two Pokemon at the same time while in the tall grass. For this reason, you will need to eliminate one of the Pokemon before you can catch the other. Come here at night and there will be a 20% chance of discovering Misdreavus.
The second position you can check out is a little further on in the game. On your way to Solaceon Town, you will encounter the Lost Tower on Route 209. Enter and search around to find Misdreavus among the other types of ghosts inside. Don't forget to take the Forza TM while you are there.
Finally, you can find Misdreavus in the Grand Underground. You will look for Dazzling Cave or Stargleam Cavern. Misdreavus is a fairly common spawn in these places, so it won't take long to find one.