If you are comfortable with the command line, gibMacOS (by CorpNewt on GitHub) is the gold standard. It scrapes Apple's own software catalog servers.
If you have a Mac that supports Sierra, the safest way is through the Mac App Store or Apple’s hidden support pages. Apple keeps older installers available for users needing to reinstall. download mac os sierra 10.12 iso and dmg image for free
Use gibMacOS or the DosDude patcher to pull the installer directly from Apple, then convert it yourself. It takes an extra ten minutes, but you will know your system is clean. If you are comfortable with the command line,