Non-repeatable
- Mali City's flea market for $3000 (Ultra Sun only; after beating Nanu's trial
- In the north of Ancient Poni Path, in an alcove west of the Vast Poni Canyon entrance
Repeatable
- Isle Aphun on Poke Pelago (Brilliant-Stone Hunting, Interesting-Item Hunting)
I'm not sure why the other answer thinks Chansey/Blissey can hold them. This has never been true.
Shiny Stone (Bulbapedia)