Wishing Pieces allow you to fight Dynamax and Gigantamax Pokemon in Wild Area Dens. This guide will teach you how to obtain them so you can host max raid battles.
Wishing Pieces can be purchased from NPCs in the Wild Area.
Locations of people who sell a Wishing Piece for 3,000 Watts:
Each of these locations can be fast-traveled to via the flying taxi. Open the map and choose the destination. Talk to one of the staff members and choose the option to spend your Watts. There is no limit to how many Wishing Pieces can be purchased.
Update: Upon further investigation, it seems as though all of the marked areas on the map will have a staff member you can purchase a Wishing Piece from. Therefore it may not be limited to the above listed areas.
A free Wishing Piece is awarded to players after finishing the game in the Wyndon Stadium. Interact with the Ball Guy (he gives different poke ball throughout the story) and he will give you one Wishing Piece.
It’s not common but wishing pieces can also be found from collecting sparkles in the overworld.