Of course, there are many unofficial sources but the files have often been cracked, tampered with or altered in some way. However, it’s more difficult to source official install media for older operating systems like Windows 7. It’s easy to get hold of a Windows 10 ISO image, burn it to DVD or write to USB and do a clean install. It’s also a good idea to clean install Windows 10 after upgrading so you are not transferring problems from your old Windows install. Simply using Windows over time will eventually degrade its performance. It doesn’t really matter what version of Windows you have, the best way to get your system working at its full speed and as issue free as possible is with a clean install of the operating system.