I know you can exchange coins for it at the Game Corner. But I think one can be found as well. According to serebii.net, there's supposed to be one "above the Power Plant". However, there is no Power Plant in Platinum. So where is it?
Actually, Serebii is not completely incorrect. The Power Plant is a direct translation from the Japanese name. In the actual game, it is called Valley Windworks. You're supposed to surf from the river on route 205 in order to get the TM that is behind Valley Windworks.