Amazon.com: Mayflash GameCube Controller Adapter for Wii U, PC USB and Switch, 4 Port : Video Games
GameCube controller support comes to Nintendo Switch - Polygon
Amazon.com: Gamecube Adapter for Nintendo Switch Gamecube Controller Adapter and WII U and PC, Super Smash Bros Gamecube Controller Adapter. Support Turbo and Vibration Features with 180cm Long Cable : Video Games
Ultimate GameCube Adapter by Panda Hardware — Kickstarter