iMovie for older Mac Mini on High Sierra

I have an old (early 2009) Mac Mini that can't run newer versions of macOS (it's currently running High Sierra 10.13.6)

The Apple App Store gives me the error: "We could not complete your purchase. iMovie can't be installed because macOS version 10.14.6 or later is required."

I have not previously ever used iMovie from my Apple account... where are you supposed to download a version of iMovie that will run on High Sierra?

Any help would be greatly appreciated. Thanks.

-Ryan