You can get one near a rock in the Dusty Bowl. Alternatively, Solrock has a 5% chance of holding one in the wild, so you can get multiple.
Hope I helped!
Source
You can get one also in Lake of Outrage (land portion) the circle of stones produces different types of stones and the chances of you getting a sun stone.
The Sun Stone can be found in the Dusty Bowl in Sword and Shield.