Unless you're nostalgic for wires, this is the best controller option available.

While most of Nintendo's attempts to repackage your childhood memories in a wonderfully nostalgic new bundle called the NES Classic Edition worked out quite well, there are some aspects of the system which don't leave retro gamers feeling warm and fuzzy.
The system's most notable shortcoming is its unbearably short controller cords. While these short controllers do accurately represent the original design of the NES, they mostly prove to be a painful reminder of how everything old-school isn't necessarily the best.
You can just buy third-party controllers with longer cords, but now, there is a far more elegant solution to this problem available.
8bitdo, the company behind the Bluetooth SNES Retro Retro Receiver we reported on earlier this year, have recently confirmed that they are producing a Retro Receiver that will work with the NES Classic Edition. If you own a previous build of the Retro Receiver, it will not be compatible with the NES Classic Edition, but the upcoming model will be.
Much like with previous retro consoles the Retro Receiver worked with, all you need to do is plug the receiver into the NES Classic Edition's controller port and connect it to any compatible Bluetooth wireless controller. At the moment, compatible controllers include certain modern-day offerings such as the PS4 controller and even the Wiimote, but if you want to have the best experience possible, you're going to want to splurge on 8bitdo's own wireless recreation of the NES controller.
Now for some of the bad news. As noted by Gizmodo, these updated receivers will not be shipping until mid-December. You can pre-order it now, but you will have to buy the $40 model that comes bundled with the wireless NES controller. While you can plug in two Retro Receivers to enjoy multiplayer NES Classic Edition showdowns, supplies are limited on receivers and controllers at the moment, so you'll want to work fast.