Windows 2012 R2 Essentials: http://download.microsoft.com/download/8/F/7/8F7024D2-AB2A-4BE2-8406-1E3AC49C5C1F/9600.16384.WINBLUE_RTM.130821-1623_X64FRE_SERVER_SOLUTION_EN-US-IRM_SSSO_X64FRE_EN-US_DV5.ISO
Windows 2016: https://software-download.microsoft.com/download/pr/Windows_Server_2016_Datacenter_EVAL_en-us_14393_refresh.ISO
Windows 2019 Essentials: https://software-download.microsoft.com/download/pr/17763.737.190906-2324.rs5_release_svc_refresh_SERVERESSENTIALS_OEM_x64FRE_en-us_1.iso