Here's the shaft you will need.
http://www2.towerhobbies.com/cgi-bin...?&I=LXERF8&P=7
And here's the plate you need. This is the complete back assembly.
http://www.horizonhobby.com/Search/D...=g70359&CatId=

Maybe somebody will chime in with a shop that stocks the shaft!
Just remember when you install the backing plate that you will have to grind the bottom of the header flange for the plate to fit right.