[jakobegger]

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.

[pfranz]

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

[jasomill]

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.

[pfranz]

Thanks a bunch! I downloaded and stashed a copy of Sierra when it first came out, but I recently bought a new Mac that came with Sierra (likely a point release and 10.12.0 likely won't install). I can restore from a netboot, but like having something I can put onto a USB drive.

[IncRnd]

You've missed the step of looking in the hidden purchases tab. If Sierra was downloaded previously, it is going to be there. There won't be a need to get copies of the software from elsewhere.

In App Store go to Store -> View My Account -> Hidden Items/Manage.