These links may be helpful to you if you have access to a Windows 8 product key, from MSDN or purchased via postcard.
NB: you should start from system running a legal, activated copy of Windows 7. If it has not been activated, you will go through may steps and then get an error message that the download is not available in your region.
General Information and Overview: how-to geek article or c|net article
Microsoft guide to upgrading with a product-key - this can get you the initial Windows 8 ISO file, around 2.3gb: upgrade-product-key-only
In September 2014, this is where you can download a free evaluation copy of Win 8.1 Enterprise from TechNet: evalcenter
Screenshots for the Win 8 to 8.1 upgrade: pc-support article
Windows 8 product-key postcard; you have to upgrade to 8.1 yourself: Amazon $70