With Yosemite in beta, it’s worth mentioning that older versions of OS X Server are still available on the app store, if you just know where to look. You can access and purchase other versions using these links:
Server 4: https://itunes.apple.com/us/app/os-x-server/id883878097?mt=12
Server 3: https://itunes.apple.com/us/app/os-x-server-3.2.2/id714547929?mt=12
Server 2: https://itunes.apple.com/us/app/os-x-server-2.2.5/id537441259?mt=12
Server 1: If you already own OS X Lion Server from the app store then you can still access it under Purchases.
