Open (Applications > Utilities > Terminal).
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).
Search for "macOS Sierra 10.12.6 (16G29) SHA-1" on your preferred Mac legacy forum (MacRumors or InsanelyMac). Disclaimer: This article is for educational purposes. The author does not host or distribute copyrighted software. Always verify that your use of macOS aligns with Apple's software license agreement.