You can download the official tool from following link:
After downloading the setup files of Windows 10 using this tool, you can upgrade your existing OS to Windows 10 or you can create bootable USB drive or DVD to install Windows 10 later. You can also save the setup files in offline ISO file format if you wish.
Once you create installation media or save ISO file, you can use them to install Windows 10 in other computers as well.
NOTE: If you upgrade your existing Windows 7/8.1 to Windows 10 using this tool, then you can use the bootable USB drive or DVD or ISO created by this tool to perform clean installation of Windows 10. But if you don't upgrade existing OS to Windows 10 and try to direct install Windows 10 using bootable USB drive or DVD, it'll ask for Windows 10 product key which you'll not have obviously.
So first upgrade your existing Windows 7/8.1 to Windows 10 so that it can be successfully activated by Microsoft servers. After that you can perform a clean installation. If you do this, product key will still be asked at installation time but you can skip the key and Windows will be automatically activated by Microsoft after installation.
PS: This Windows 10 Media Creation Tool is same as Windows 8.1 Media Creation Tool: