You are likely having a hard time finding that part due to it being sold as a complete unit. You just need the top half, correct? You should be able to find the complete unit and just unscrew the top and attach that to your drain.
http://www.oatey.com/Channel/Shared/ProductGroupDetail/106/Cast+Iron+Flange+Replacement.html
http://www.oatey.com/Channel/Shared/ProductGroupDetail/106/Cast+Iron+Flange+Replacement.html