We can approach a computer software or hardware seller. They do sell copies of original software. So you can get licensed operating system that you want. Perhaps later after installing the operating system, you need to download from the manufacturer/supplier site, the latest updates.
The license key is important.
Also on your system itself, you may get information regarding purchase of operating system. Go to help and support menu (in the start menu) and query regarding purchase.
Go to the site that produces the desired operating system. Pay the amount online and download the OS. The license key will be given to you. Perhaps the call centers or help lines of the manufacturers also will help regarding this.