You don't catch one, but you trade one for a Ditto. There's a caravan on that route, and a scientist will offer to trade her Rotom for a Ditto, which you can get in the forest-y parts of the Giant Chasm.
Note that you do not get to Route 15 until after you've beaten the Elite 4/ Ghetsis.
Marriland, Route 15, B2W2
