You can get it on route 11. You use waterfall to climb the waterfall to the left of the trailer and the pokeball containing it will be on your right.
Overheat is on Route 11, starting from Opelucid City just head east onto Route 11 until you see a Waterfall (it comes up quick). Go up the waterfall and you'll see the TM to the right on the ground:
(Source: Guide Strats)
