Create USB Boot device on Linux or Windows
Unless you are going to go the route of downloading an illegal/pirated copy of an installer there is no way of downloading Mac OS on anything but Mac OS.

There are ways to do this, though. The first would be a friend with an older Mac, preferably of the same vintage as yours and have him download a compatible installer.

Another way would be to make a Mac Genius appointment. They will help you re-install Mac OS using installers available at the Apple store.

Also if there is a Mac user group in your area, go to a meeting and ask if someone can help with an installer.