The RTM download ISOs are available everywhere now
I downloaded the Windows 8.1 Single Language x64 ISO
With Windows 8.1, you cannot use your product key to install during setup, instead, there are a few default keys just for the sake of installation, after you enter that default product key (which cannot be activated its just for the sake of installation), when you are on your desktop, go to PC Settings, then activation, the enter either the key built into your BIOS or your retail product key and it will activate right away
if you don't kinow what is the key built into your BIOS, before you format, get it using Belarc Advisor (google it, install it, then look under software licenses it will be there)