Up until now, there's been no way to purchase an additional power supply unit for the Xbox One - not even through the Microsoft online store. Amazon.com of course, offers 3rd party and original Xbox One power supplies, but Microsoft has not presented the option.
Major Nelson announced on Reddit, that he worked with the Xbox support team for customers to purchase additional Xbox One power supply units.
Here's how to do it…
Visit https://myservice.xbox.com/ (you'll need to register your Xbox One device if you haven't already).
Choose to Replace an Accessory.
Select the device you have registered.
Choose Power Supply, and tap the Next button.
Choose the method for replacement.
For $35 you can get a replacement and ship yours back, or for $50 you can just have a new power supply unit shipped to you and you can keep your old one.
$50 is still cheaper than you can find online at Amazon.com.