Amazon offers Prime members the My Arcade GamePad Classic Wireless Controller for the NES Classic Edition Gaming System for $14.99 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, shipping costs $4.98.) That's the lowest total price we could find by $4. It can also be used with the Wii or Wii U with compatible games and requires two AA batteries (not included).
Note that it's currently on backorder with an estimated shipping time of one to two months.