If you are a repair shop or IT administrator, you may legally download the installer once per Apple ID, then deploy it across multiple eligible Macs.
On this page, scroll to the "Use a web browser" section and click the link for . It will download a file named InstallOS.dmg . How to Use the DMG File Mac Os Sierra 10.12.6 Dmg File Download
Note: If you are on a newer Mac running Mojave or later, the App Store might refuse to download Sierra. In this case, use Method 2. If you are a repair shop or IT
“Volume is not a valid volume mount point”: Mac Os Sierra 10.12.6 Dmg File Download
You can obtain the installer or specific updates directly from Apple's support pages: Full Installer (via Web Browser)