So the actual Cisco website should show you that you can search by model see all available and then see what releases in software are available go to this link hit switches or routers and follow it through to a specific device and it will show you what's currently there
http://www.cisco.com/c/en/us/support/index.html
also you can check the difference's between software versions and platforms using this
http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp
