Vole stopped providing copies of Windows 7 Professional and Windows 8.1 licences to OEMs which means that you should not be able to find one in the shops.
Two years ago, Microsoft stopped selling Windows 7 Home Basic, Windows 7 Home Premium, and Windows 7 Ultimate licenses to OEMs. With Windows 7 Professional and Windows 8.1 out of the picture Windows 10 is the only remaining option. Unless you want Linux.
The Windows Lifecycle chart for sales doesn’t have an end date for Windows 10, since that operating system doesn’t have a successor.
Mainstream Support for Windows 7 ended on January 13, 2015 while Mainstream Support for Windows 8.1 will end on January 9, 2018. In theory Windows 10 will still get it until October 13, 2020.
The Windows 10 Anniversary Update, released in August 2016 and a new version, the Windows 10 Creators Update, is slated to arrive in “early 2017”.