Where can I buy a suitable AC adapter?
Nintendo Classic Mini: NES is powered by a USB cable (type A to type micro-B) and a USB AC adapter (output: 5V/1.0A/5W), which is available from retailers.
Be sure to read the instructions of the device you are using and follow all warnings. Any device into which the USB cable is plugged must be powered on to provide power to Nintendo Classic Mini: NES.