Finding a pre-built ISO online from untrusted third-party sites carries significant security risks, such as malware injection. The safest and most reliable method is to create your own ISO file using an official installer from Apple.
Create a virtual hard disk with a minimum of space (dynamically allocated). macos high sierra 10136iso
It provides excellent stability for Macs manufactured between 2009 and 2017. Technical Specifications of macOS 10.13.6 Finding a pre-built ISO online from untrusted third-party