How to downgrade to macOS Sierra without Time Machine I upgraded to High Sierra a week ago, my MacBook Pro 2016 (15 inch) is very slow now even in normal daily usage.

I wanted to downgrade back to Sierra, but even tho my MacBook came originally with Sierra, I cannot find it in "App Store > Purchases", there is only Mavericks which I bought on my previous MacBook.

Is there any way I can download Sierra from Apple? Is there any other trustable source?

Or maybe I would see Sierra in App Store if I re-install "OS X Mavericks" again?
I got it from this link.
IMPORTANT:
DO NOT downgrade using macOS Sierra (10.12 Installer) disk utility to Erase your disk, you HAVE TO use the Online Recovery tool (or High Sierra (10.13) Disk Utility) to Erase your disk from APFS back to HFS+ , then restart and use your macOS Sierra installer from USB.
If you use Sierra to Erase disk, you'll get stuck in a Reboot Loop which is what happened to me.