I so wish Apple had not removed the older Sierra installer from the Mac AppStore... I need to update some machines but High Sierra does not seem ready for reliable use yet.

If you've downloaded Sierra on another Mac before, you can still find the Sierra installer in the Mac App Store in the "Purchases" tab.

If you don't have it in your purchases list, you can just ask a friend who has it to download Sierra for you from the Mac App Store.

Then you can just copy the Installer app on a thumb drive and run it on your Mac.

Only problem might be that maybe you can't download Sierra from a machine running High Sierra...

UPDATE: I'm wrong, see comment below.

They stopped doing that with Sierra:

"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 the current version of macOS."

https://support.apple.com/en-us/HT201272

Note, however, that if[1], and apparently only if[2], you’ve downloaded Sierra from the App Store in the past, you can install

https://github.com/mas-cli/mas

and then use the command

    mas install 1127487414
to download the latest Sierra installer from the App Store.

[1] It worked for me.

[2] Or so I’ve heard.