You should probably forget about the body until you learn to fly really well ... either that or buy some stock in Smartech...

The main drive gear? Try here... http://www.raidentech.com/19tagebig.html

While you are ordering... you should probably get some other parts also: Main rotor blades, for sure ... tail rotor, probably ... tail motor, so I have heard ... landing gear and the 4 struts that stick into it....

Ideally.... You could have a replacement part on hand for EVERYTHING! [:-]