Apple still provides links to download older versions of macOS through their support site. This is the safest method to ensure you aren't downloading a file with malware. For macOS High Sierra (10.13): You can access the macOS High Sierra download page
If you accidentally install Sierra when you meant High Sierra, your APFS drives will not be readable. Download Macos High Sierra 10.12 Dmg
The dusty 2011 MacBook Pro sat on Elias’s workbench like a relic from another era. Its screen remained dark, a victim of a botched update that had left it stuck in a boot loop. To revive it, Elias knew he needed a specific digital key: a bootable file. Apple still provides links to download older versions
Open Terminal on a Mac (preferably an older one) and run: Download Macos High Sierra 10.12 Dmg