Shard is found inside The Cave of Trophies solo dungeon. Entrance to this dungeon is at the end of a main road that goes northeast of The Hollow City. Mobs that are level 47 fill the dungeon and sometimes come in groups of three.
In order to collect the collectible you need to reach the southern part of the cave. Shard is on ground, between two large mammoth skulls. One of this skulls has large tusks.
Find and enter The Cave of Trophies solo dungeon.
Dungeon entrance map location.
Sand Kingdom Moons - Super Mario Odyssey Power Moons