Amazon.com: Brook Gamecube to Wii U Controller Adapter - Support Gamecube Controllers Adapter for Wii U and PC USB Android, up to 8 GC Controllers in The Same time (MAX), Turbo fire
GitHub - shockdude/gcn-usb-adapter: A vJoy driver for the WiiU Gamecube USB adapter, but hacked to support PS3 turntables.
Here's how to get Wii U's GameCube Controller Adapter working on Windows | VentureBeat
Amazon.com: CLOUDREAM Gamecube Controller Adapter for Switch Gamecube Adapter Wii U and PC, Super Smash Bros Choice Support Four Gamecube Controller and 70IN Long Cable : Video Games
Massive's gamecube adapter driver dev blog | Wii U GCN USB Driver
TTX Tech GameCube Controller Adapter for Wii U and Switch - Micro Center
4-Port Controller Adapter for Nintendo GameCube to Switch & Wii U & PC | eBay
Portholic Gamecube Controller Adapter with 4 Ports Compatible with Wii U, Nintendo Switch for Super Smash Bros Ultimate (No Drivers Needed) - Black - Newegg.com
4 Port GameCube Controller Adapter for Nintendo Switch Wii U PC Super Smash Bros - Walmart.com