You Can Pre-Order Switch's NES Controllers Now
But remember, there's a catch.
To pre-order these controllers, you need to have a subscription to Nintendo's new Switch Online service. You can't sign up for the free seven-day trial and pre-order the controllers; you need to be paying for a subscription. The subscriptions cost $4 / £3.50 for one month, $8 / £7 for three months, and $20 / £18 for 12 months--which you can pay for with My Nintendo points if you prefer.
These NES controllers also don't work like regular Joy-Cons. They can only be used to play Switch Online's exclusive library of classic NES games, such as Metroid, Super Mario Bros., and The Legend of Zelda. The NES controllers cannot be used while playing in handheld mode either, so your Switch will need to be in kickstand mode or docked and connected to a TV. The NES controllers do at least charge when connected to the Switch in handheld mode.
The NES controllers are sold in pairs of two and cost $60 / £50. You'll be able to use both controllers to play couch co-op in most of the NES games that come included in Switch Online. For some of the single-player games that don't support cooperative play, the second person can point out enemies, hidden areas, and items for their partner.
The NES Switch controllers will start shipping out in mid-December 2018.