Leopard in VMWare Fusion
for about $20 USD you can buy Mac OS X 10.6 Server from Apple.  I did so just the other day ordering by phone.

This version of OS X IS allowed to be run as a Virtual Machine as long as the host is Apple hardware.

So this will allow you, for example, to run 10.6 in a virtual machine on a modern iMac running Mac OSX 10.11.