I was just wishing today to install an old version of Garage Band that could run on an older Mac. But I don’t have it on a disc, and can’t see how to download old versions.
Do you own Garageband on the Mac App Store? On the older Mac, download Garageband from the App Store's "purchased" page. You will be given an option to install the last compatible version. Note that you absolutely must do this from the purchased page of the App Store and not anywhere else!