@gtwilson I have found an official Apple page which includes this gem: To reinstall Safari, Mail, the App Store, or other apps that are included with the Mac operating system, reinstall macOS. macOS Sierra or later doesn't appear in the Purchased tab. Instead, use the Search field in the upper-right corner to find and open the App Store page for that macOS.
But now that Sierra has been superseded, only High Sierra is available.
WAIT! I may have found something on a forum, it's a package of 4.77 GB, claiming to be the absent Sierra downloader, I should have it available in 25 minutes & will check it out then. I do know that my saved Sierra installer app is 4.78 GB, so it's a possibility.
// @indigo