How to obtain an El Capitan 10.11.6 .ISO file

Hello All,

I have been learning and taking courses about setting up virtual machines and I have been primarily using VM VirtualBox by Oracle. I have set up a couple different windows VMs, but I wanted to tryout setting up an OS X VM. I did a little bit of looking around for an OS X 10.11.6 (El Capitan) .ISO file to use but wasn't able to come to any conclusions that appeared legitimate.

Where would be the best place to find it? Or is it even possible to obtain?

Thanks!