A dusk stone can be found on Route 224, the Galactic Headquarters in Veilstone, and the Wayward cave. Also, if you have a pokemon with pickup that is level 31-40, it may obtain it.
A dawn stone can be found on Route 225, Mt. Coronet, Route 212 (it's not visible so you need to use the Drowsing App). You also can get it with a pokemon with Pick-up that is level 51-60.