Microsoft Download link:

Download the OS and perform a clean install. You will be prompted for the product key during install, but not for the download. Use the same Win 7 or 8 key that you legally own.

The same download can be used for multiple installations as long as you have unique legal Win 7 or 8 keys.

Win 10 Home or Pro will install depending on your product key, the same download is valid for both.