I was in the same boat, Leatherhide. I disagree with what others are saying as you and I both navigated to the Windows Store and purchased full versions of Windows 8.1. We are not trying to do anything illegal here, we simply want to install the product we paid for!
Bootcamp assistant won't recognize the link that we download from Microsoft, since it is only executable on a PC, hence our dillema. Again, we are people who bought this software directly from Microsoft, and Microsoft won't give us the ISO!
Anyway, sign up for this trial program and you will get the ISO download. After you install it via Bootcamp you can register your own unique product key to activate it.
What I did, was install the trial version just so I could use my legit Microsoft download to create the ISO for my personal copy of Windows, then wiped everything an installed from my own ISO.
I hope this helps as I had to comb the internet for hours to figure this out.
Link:
http://technet.microsoft.com/en-us/evalcenter/dn237247