>>17508843There's three of them in total:
You find the first one in that "No... You're not the one..." building when you're deemed "The one.".
You get given the second by some NPC in the hotels, it's time-based so it takes 3-4 days to get it if I recall correctly.
The third one is hidden in that forest next to the Pokémon village, it's in the area with the hex maniac.
Give them to Sycamore and choose either the Latiasite or Latiosite.
Latiosite masterace. 
