Wii U can use two GameCube controller adapters
Gamecube Controller Adapter for Wii U - Solaris Japan
Amazon.co.jp: メイフラッシュ ニンテンドーGameCubeコントローラ・アダプタ(Wii/Wii U用) : ゲーム
限定販売】 ゲームキューブ コントローラー用 接続タップ Switch WiiU PC スイッチ _ zppsu.edu.ph
Amazon.com: Super Smash Bros. GameCube Adapter for Wii U : Nintendo Gamecube Controller Adapter: Video Games
駿河屋 - 【買取】GC Controller Adapter for Wii/Wii U（WiiU）
駿河屋 -<中古>GameCube Controller Adapter for Wii ＆ Wii U（WiiU）
Gamecube Controller Adapter - GameStop.ca
Mayflash GameCube Controller Adapter for Wii & Wii U (Dual Pack), White - Walmart.com
GameCube Controller Adapter Announced for Wii U - IGN News - YouTube
Wii U Gamecube Controller Adapter〔USA〕 Brook Gaming - Official Brook Gaming Store
GameCube Controller Adapter for Wii & Wii U (Dual Pack), White
GameCube Controller Adapter for Wii U to return to select Australian Stores - Vooks
Buy Gamecube Controller Adapter for WIIU | retroplace
Amazon.com: Mayflash GameCube Controller Adapter for Wii U, PC USB and Switch, 4 Port : Video Games
Confirmed: The Wii U GameCube Controller Adapter will not work on DDR, Active Life, or Cyberbike | Gaming.Fit
Gamecube Controller Adapter for Wii U BX-W201のeBay公認海外通販｜セカイモン
Nintendo Official Wii U GameCube Controller Adapter BRAND NEW SEALED! | eBay
TTX Tech GameCube Controller Adapter for Wii U and Switch - Micro Center
Hands On: Wii U GameCube Controller Adapter | Nintendo Life
MayFlash Gamecube to Wii & Wii U Adapter - YouTube
