Is there a "permalink" for Sierra at the App Store?
Yes - Contrary to past OS releases, Apple has an article with links to the store and detailed requirements for getting and installing macOS Sierra (even after High Sierra is out).


https://support.apple.com/en-us/HT208202 -  How to download macOS Sierra
macOS Sierra Installer on the Mac App Store


I would use the help article HT20802 to check for an updated link if the Mac App Store one fails you, but the one above is live now on the store.