If you have access to a Mac and have downloaded the "Install OS X El Capitan.app" from the official link above, you can create a bootable ISO using these Terminal commands: Create a blank disk image
hdiutil create -o /tmp/ElCapitan -size 6500m -volname ElCap -layout SPUD -fs JHFS+ Mac Os X 10.11 El Capitan Iso Download
Since Apple provides a .dmg or .pkg file, you must convert it if you specifically need an ISO for a virtual machine like VirtualBox or VMware: OS X El Capitan - Technical Specifications - Apple Support If you have access to a Mac and
: Allowed users to automatically position two applications side-by-side in full-screen mode, similar to Windows' snap feature. System Integrity Protection (SIP) Use Terminal to Create a Disk Image
Ensure the "Install OS X El Capitan" app is in your Applications folder. 2. Use Terminal to Create a Disk Image