Depending on your system architecture, use the appropriate link from the official ESET help pages :
If you specifically require an older version (e.g., ESET Smart Security Premium v14 or v15) for compatibility reasons, you must access the ESET Knowledge Base article for previous versions.
For a true offline installer, you must use the package available on the ESET download page. This file contains the full installer and does not require an internet connection during the installation phase (unlike the small web installer stub).