You can find Rotoms, of all different forms in the trash cans. You can also change a Rotom's form on the second floor of Professor Sycamore's lab. The twist to catching Rotom is, it's only on Tuesdays. To access the different sides if the bars, you need to go into different entrances. The two ruins with staircases leading down are on opposite sides of the East-West path that leads into the connecting tunnel between Route 15 and the snowy town.
My first post, hope it helps,