Obtain the Kangaskhanite.
XY: Go to Ambreat Town and go to the Glittering Caves. You find the Kangaskhanite on the right as you enter the cave.
ORAS: Found at Pacifildog Town. It's behind the sign next to the Pokemon Center.
SM/USUM: Get it in the Battle Tree for 64 BP.
Let's Go: Get it at the Indigo Plateau for 30,000.