Rotom, can be found in the Old Chateau in Eterna Forest. If you've been inside, you've might have seen a ghost or two there. Now you must have the Nationel Pokedex to get Rotom. When you go to Eterna Forest the Gym leader of Eterna (she'll be at the front of the Old Chateau) she'll tell you about a mysterious pokemon found inside, she'll than leave.

Go inside to the secound floor, than go through the center room, than go to the secound last room on the left. They'll be a tv inside, go up to it and talk to it. It'll say theres a pokemon visible inside. It'll than give you the option of thumping the tv, put yes and you'll enter a battle with Rotom. Its level 15 so it should be easy to catch with a couple of ultra balls.

If it doesn't work when you go to the tv, you'll need to see the two ghosts of the Old Chateau first. The first one is on the first floor. When you enter the Old Chateau, talk to the statue near the center room, than enter the center room. You should see a ghost of an old man across the room, it'll than float away.

The second ghost can be found on the secounf floor. Go up stairs to the center room. Head to the seound last room of the right and enter. You should see a girl in the other room, she'll then leave the room. Leave the room and than go back to the tv room. You should now be able to get Rotom.