Till now, there was no direct way to buy an extra power brick from Microsoft. Microsoft was aware of the need of the consumers and they have now made it available for purchase online from Xbox site. Major Nelson revealed this info on Reddit today.

Hey gang, I have an update on this. I have worked w/ the Support team and we’ve enabled the option to purchase additional Xbox One power supply units via https://myservice.xbox.com

I have not had a chance to check it out myself yet – but the team just emailed me to let me know they have added that option to the online tool.

Thanks!

Larry