You can buy it from MS If you want, or you can buy it from Newegg, Amazon, Etc. All you need is a Windows key, then you can burn a win8 CD or USB drive all you want.
You can create a Windows 8.1 USB drive with Microsoft's tool here, just match the langauge and edition (8.1, 8.1 pro, etc) to the type of Windows key you bought: http://windows.microsoft.com/en-us/windows-8/create-reset-refresh-media