Where can I buy Windows 8 Pro product key directly online?

I've already upgraded to Windows 8. But I want to buy another upgrade license for my friend and his Windows 7 is currently broken and needs re-installation which I don't want to bother with.

Can I buy a upgrade key for him without need for him to re-install Windows 7 and then the run the upgrade assistant?

I just want to buy the key, burn the iso(which i downloaded) into a usb stick and ask him to install directly from the USB stick replacing the Windows 7. Let me know if it's possible. Alternate option is to buy the pricier 69.99$ disc from a store.