Step 1 of the Installation Guide:
The full operating system is a free download for anyone who has purchased Mac OS X Snow Leopard, Lion, or Mountain Lion or has a Mac preloaded with OS X Mavericks, Yosemite, El Capitan, or macOS Sierra. Download the Application from the Mac App Store using your Apple ID on any Mac or functional computer running OS X 10.7.5 or later.
You need to get macOS / OS X only from Apple or you are in violation of the site rules. Not doing so will get you an instant lifetime ban from the community. Obviously, not a good choice. VM installs of macOS on a PC are also not allowed.
You can borrow a friend's Mac to download it or buy the Snow Leopard DVD to install if your hardware is Haswell or older. Then you can use iBoot for the install from the Snow Leopard DVD. Apple is still offering El Capitan for free, so you would download and install that from your Snow Leopard installation on your hardware. After that you can get Sierra if you want that.
