You can use the Smash Brothers Gamecube adapter in portable mode with a USB C adapter (Rumble works too) : r/NintendoSwitch
Amazon.com: GameCube Controller Adapter : Video Games
Does the USB to gamecube adapter work in Wii mode on the Wii U? : r/wiiu
Just found out the Wii-U adapter works on switch! It's time to get serious about smash. : r/gaming
Mayflash GameCube adapter not working : r/NintendoSwitch
Amazon.com: Mayflash GameCube Controller Adapter for Wii U, PC USB and Switch, 4 Port : Video Games
The GameCube adapater works in handheld mode too! : r/NintendoSwitch
So you only need to plug is one USB(the black one) to get the GameCube controller adapter to work with all 4 controllers. (X-Post from /r/Nintendo) : r/smashbros
Will this specific adapter work with Dolphin? It's a Nintendo Switch Gamecube adapter and appears to be a first party one : r/DolphinEmulator
With a few hitches, info in comments] Mayflash's Gamecube Controller Adapter now works with the Switch : r/smashbros
Mayflash GameCube Switch/Wiiu Adapter working onPS3 - YouTube
Mayflash 4 Port GameCube Adapter - In Game Reset is not working | GBAtemp.net - The Independent Video Game Community
Having trouble with the old Mayflash GC to USB adapter and getting it to work with newer versions of Dolphin : r/SSBPM
GameCube controller issue? Characters moving straight down when I'm not even touching the controller. If anyone has a solution it'd be appreciated : r/DolphinEmulator
Are the third party Wii U-Gamecube controller adapters good? : r/smashbros
Switch Dock Mod Adds Built-In GameCube Controller Ports | Nintendo Life
Help, which Mayflash driver do I need for my Gamecube adapter? : r/DolphinEmulator
Mayflash Gamecube Adapter for RetroArch? : r/RetroArch