There's 3rd party ones, but I did some Googling and MS might even just send you one if you follow these steps:
"1. You will need a contact in the countries that support repairs(in this case UK)
2. You will need to create a Microsoft account at accounthttps://signup.live.com for your UK contact, his name his adress and UK country region(basically with his details) he does not need to have access to the account
3. You will need to register the console at
https://myservice.xbox.com/
4. You will need to create a accessory replacement order for the Power Supply Unit(PSU)
5. You will then need to ship it off to the service center, you can print the label with the address from the order page
6. Once the PSU is received a replacement will be sent to the UK address, the PSU will have a UK power plug so an adapter will be needed
7. Also your UK contact will nedd to send you the new PSU unit
8. Have fun with your Xbox One!"
Obviously, if you're in the UK, it's a lot simpler. That was how someone got it when they were in Sweden and bought a US console.
EDIT: If you have an old non-slim 360 power brick that says 203w on the label of it, you can just buy a pinout converter too.