how to buy windows 8 for use with fusion?

I'm running VMware Fusion 5.0.3 on a Mac that is running OSX 10.8.3. I am currently running Windows XP. I would like to buy Windows 8 via download and create a new VM. When I look at the Microsoft website, I see an option for upgrading my XP on the same computer (in my case, in the same VM). What is the right way for me to buy Windows 8 so that I can use it to create a new VM?