UPDATE
You might want to read my newer post about this before proceeding …

===

Quick guide to something which is unfortunately and unnecessarily hidden and convoluted.

If you want to buy a new Microsoft OEM power supply for the Xbox One, whether because yours broke or you moved to a country with power settings than the one from where you purchased your console, here’s how you do it.

* Go to Xbox Online Service Centre.
* Sign in or register with a Microsoft account
* Register your Xbox if you haven’t done so already before
* Click on replace an accessory, select the xbox (not the kinect), and click power supply!

You can probably handle it from here. They say you will get the correct power supply for the shipping address you use (i.e. if you ship it to a UK address you will get an UK one).

In the UK this costs £30 at the time of this writing. Getting it to you can take 2-3 weeks.

If you have any other questions, contact Microsoft 😛