Source: GameSpot, Nintendo's Japanese website (for the photo)Super Smash Bros. Ultimate is coming to Switch this December, and it will support a variety of controller options, including the GameCube controller--the preferred gamepad of many Smash Bros. players. Nintendo confirmed during E3 2018 that Wii U's GameCube Controller Adapter will be compatible with Ultimate, but if you don't have one of those laying around, you'll be able to pick up a new one at launch.
During a hands-on session with Super Smash Bros. Ultimate, a Nintendo representative said that a new GameCube controller adapter will be released alongside the game, and it'll retail for $20. Nintendo also confirmed it will offer a new line of Smash-branded GameCube controllers later this year, although pricing details for those have yet to be announced.
UPDATE: The GameCube controller will cost $30 (special thanks to Wario64 for finding this on Amazon).