That's what I meant: I'm pretty sure digitalriver.com is Microsoft's own CDN, used for App Store and MSDN downloads. You can even verify the hashes on an actual Microsoft subsite — possibly here? http://msdn.microsoft.com/en-us/subscriptions/downloads/
You can also get an upgrade application from http://windows.microsoft.com/en-US/windows-8/upgrade-product... to move Win7 up to Win8.
But AFAIK you can't get a MS Windows 8 ISO to download from Microsoft itself?