Earlier quoted context omitted.
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 mac…
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
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.