GarageBand is only available from the AppStore, since Apple moved it to the AppStore. All download links to updaters have been removed from Apple'S download page.
And you can only purchase applications from the App Store, if the App Store still offers a compatible version for your system. On older system versions you can only reinstall applications you purchased previously.
if you ever purchased GarageBand from the AppSore with your AppleID, and you are seeing it in your Purchase list (the fourth tab of the AppStore) you should be able to reinstall it from the Purchase tab.