You can get them by trading members from the Green Room for them.
• 4★ character: Wish Jewel x1
• 3★ character: Wish Piece x50
• 2★ character: Wish Piece x5
Note that you will permanently lose any characters you trade.
To access the Green Room, go to Characters > Green Room.
Wish Pieces can also be obtained from Virtual Shows based on the amount of Fan Points you earn.
The number of Wish Pieces you receive will be displayed at the end of the Virtual Show.