How to download Sierra DMG file if mac can't support it How do I download a .dmg file or Sierra (or any recent macOS)?

I have one very broken MacBook pro, and I want to make a bootable usb drive to try and fix it. I have another, older, MacBook, which unfortunately does not support Sierra, but it is the only macOS that I can find in the app store. It won't let me download it, because it doesn't run on my MacBook.

I want to either force the app store to let me download it (on my MacBook, even though it can't run it), or I want to download the file from somewhere else.

Is there no way that I can just download Sierra on my Mac that does not support it?
The only legal way to download it is probably to make a usb drive on a friends Macbook that supports Sierra. Otherwise you can try to reinstall macOS through the recovery if your MacBook Pro supports Sierra.