If you are downloading Sierra to install on an unsupported Mac (i.e., a Mac that Apple never intended to run Sierra), the community-standard tool is the DosDude1 macOS Sierra Patcher.

While this tool is designed to create bootable USB drives, it also allows you to download a genuine copy of Sierra directly from Apple's content delivery network (CDN).

Step-by-step to get the genuine Sierra installer:

You now have the genuine, unmodified Sierra 10.12.6 installer. Let’s turn it into an ISO.

Your ISO is not just for VMs. You can flash it to a USB drive to install Sierra on a bricked Mac.

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia
--volume /Volumes/ISO --nointeraction

If you do not own a Mac and cannot run the Terminal commands above, you need a trusted third-party ISO. The most widely trusted source in the virtualization community is the Internet Archive (archive.org).

Why Archive.org?

Search string for your browser: "macOS Sierra 10.12.6 ISO" site:archive.org

Look for the following credentials in the description:

Warning: Even on Archive.org, avoid "Hackintosh distros" (e.g., Niresh, iAtkos, Olarila). They are heavily modified and insecure. Only download "Vanilla" (unmodified) releases.