You're looking for a Windows Server 2016 ISO file. Here are some official and safe ways to obtain it:
To install Windows Server 2016 from the ISO file, follow these steps: windows server 2016 iso
If you download the 180-day evaluation ISO from Microsoft, you cannot convert it to a full retail license via a simple key change (DISM often fails). You must perform an "Edition Upgrade" via PowerShell or reinstall. Save yourself the headache: use your VLSC login. You're looking for a Windows Server 2016 ISO file
: This is the most common source for a free 180-day evaluation version of Windows Server 2016 Datacenter or Standard. This version is fully functional but will expire after the trial period unless activated with a valid retail or volume license key. windows server 2016 iso