How can I download the installer for macOS Sierra 10.12?
Apple now has a page up and live store links to download Sierra after High Sierra was announced.

Apple makes Sierra available even after High Sierra is released.


https://support.apple.com/en-us/HT208202


The link to the installer on the Mac App Store is: https://itunes.apple.com/us/app/macos-sierra/id1127487414?ls=1&amp;mt=12

If for some reason that article or link are gone, you may need to have an Apple ID that previously downloaded 10.12 Sierra - then it will be in purchased and a quick download. If you feel it’s missing, check with Apple Store support and your “purchase” receipt emails / purchase history. This is a change from older releases where you had to make sure and get an installer purchased before general release.

If Sierra legitimately isn’t in your purchased list, and for some reason the KB above gets pulled at some point:


You might be able to take your Mac to Apple Service and have the OS loaded.
You might be able to use Internet Recovery to install the OS that came with your Mac - so you’d need to buy or have a Mac that shipped with 10.12.
Find someone you trust to make you a bootable Install macOS Sierra installer drive - a 8 GB USB drive works well for this.