While as standard you cannot get older versions of Server.app and also as standard cannot get Snow Leopard Server anymore if you really must there are legal ways. This unlike Lion, Mountain Lion, Mavericks, Yosemite and El Capitan does not use the current Server.app approach it is instead a special server version of OS X. If you an original and now very old copy of the Snow Leopard Server DVD then you could also install that in to a fresh empty VM in VMware Fusion. The only version of Server.app you can current buy and download is the version that requires running in El Capitan and is currently Server.app 5.1

You then run Server.app inside the VM - Server.app will then configure that copy of OS X to act as a serverĭifferent versions of OS X need different versions of Server.app.

You then copy the Server.app you purchased in to the VM.

You use VMware Fusion to install a copy of OS X in to the VM.

You install VMware Fusion in to your Macs existing copy of OS X.

As some additional clarification to Leopardus' reply.

Os x el capitan vmware image