Before searching for a download link, it is crucial to understand the risks. Many websites offering "free" versions of Windows 7 or Vista bundle the installation files with malware, keyloggers, or backdoors. Only download ISO files from reputable archives.
These OS versions do not natively support USB 3.0. If your keyboard and mouse stop working during the installer, you will need to use a USB 2.0 port or inject USB 3.0 drivers into the boot image.
Windows Vista is significantly harder to find than Windows 7. Because it was less popular, fewer official digital copies were ever distributed.