Hello, hughmannface.
Yes, you can download the High Sierra update with another Mac that is capable of running High Sierra - one that did NOT originally ship with Mojave or Catalina. You can download the installer at the link provided below and carry it over to your other Mac via USB drive or flash drive.
These Mac models are compatible with macOS High Sierra:
• MacBook (Late 2009 or newer)
• MacBook Pro (Mid 2010 or newer)
• MacBook Air (Late 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2009 or newer)
• Mac Pro (Mid 2010 or newer)
✅ - Download the macOS 10.13 High Sierra upgrade installer here:
https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12
(Don’t use FireFox for this link)