Where to get the installer safely
If the initial download is base Sierra (10.12), you can update to the final version using these direct standalone update links from Apple: macOS Sierra 10.12.6 Update (for users already on 10.12.5).
Once you have the Install macOS Sierra.app , you can create a bootable USB drive to install the OS on a target Mac.
To legally secure an offline installer (DMG) for macOS Sierra 10.12.6, users typically follow these steps: Official Support Links
If you are searching for the you likely fall into one of three categories:
If you are already running an older version of Sierra (like 10.12.5) and just need to update to 10.12.6, you can download the standalone update packages: