Here's the current GM replacement... http://www.heartbeatcity.com/store/product/13569/Camaro-Fan-clutch--302--396--%26-AC--67-68/
The last used, original one I saw sold for over $1100.00 with a broken cooling fin.
Kurt has a good photo of an original '68 Z28 fan clutch along with more info on the subject here... http://www.camaros.org/coolingsystems.shtml#fan
Paul