How to Connect a GameCube Controller to the Wii - Howchoo
Mario Kart Wii CTGP - Using Gamecube controllers on Wii U for MKWii - YouTube
Wii U can use two GameCube controller adapters
How to Use a Gamecube Controller on a Wii: 5 Steps (with Pictures)
Amazon.com: Y Team Controller Adapter for Gamecube, Compatible with Nintendo Switch, Super Smash Bros Switch Gamecube Adapter for WII U/PC with 4 Port : Video Games
GameCube to WII and WII U Dual Pack Controller Adapter – Flashback Limited - Repair, Replay, Relive