Apple provides the .app file, but virtual machines require an .iso . You can create your own using the to ensure the file is clean and safe. Step-by-Step Terminal Commands:
hdiutil create -o /tmp/ElCapitan -size 8000m -layout SPUD -fs HFS+J os x el capitan iso
If you have downloaded the official installer from Apple (which results in a file named Install OS X El Capitan.app in your Applications folder), you can convert it into an ISO using the Terminal on a Mac. Apple provides the