Next to Windows XP, Windows 7 is an extremely popular version of Windows and continues to be used on just over 48% of Windows based desktops globally according to Net Applications data for the month of October 2016.
By comparison Windows 8/8.1, which never really gained a lot of traction among consumers or Enterprise customers, is only showing up on about 10.5% of the same collection of tracked desktop systems by Net Applications last month.
Microsoft stopped retail sales of Windows 7 Professional back on October 31, 2013 while they stopped the same sales of Windows 8 and 8.1 on October 31, 2014 and September 1, 2015 respectively.
These dates are part of Microsoft's lifecycle for those products.
After retail sales of an OS are stopped the final opportunity to sell those items is left with OEMs, Other Equipment Manufacturers, and they usually sell the OS with new hardware or pre-installed on a new computer.
Well both Windows 7 Professional and Windows 8.1 have now reached their end of OEM sales date which was this past Monday on 31 October 2016. That means there are no longer any legal methods for purchasing these two OS's. Understand that not being legally allowed does not mean there are not some OEMs out there still selling them.
My recommendation is to avoid those channels because if you are a company/enterprise in the Windows ecosystem the last thing you want is to muddle up your licensing by purchasing products that are no longer allowed to be sold.
Now, the end of OEM sales does not mean the end of support from Microsoft.
Based on the lifecycle support page Windows 7, which must have Service Pack 1 installed, is in extended support until 14 January 2020 and Windows 8.1 is in mainstream support through 09 January 2018 and then extended support until 10 January 2023.
This change also means that the only desktop version of Windows that is available for purchase by consumers/enterprise customers is Windows 10 which, according to those same Net Application numbers from October, is installed on about 22.6% of global Windows machines they track.