Option to buy Windows 8.1 Pro Pack not Available

I have Windows 8.1 Single Language installed on my laptop. Since my desktop which is Windows 8.1 Pro with WMC is already upgraded to Windows 10, it loses the Media Center program after upgrading. I want to have the Media Center so I think about buying the Pro Pack to add WMC to my Single Language. But when I go to Add features to Windows 8.1, the option to buy product key is no longer available. Is there other way to upgrade my Single Language to Pro with WMC?