You can find that exact derailleur on Ebay, sometimes NOS. But they seem to run expensive -$35-125 or so. I'd get a different derailler. There are plenty that will work: others here will know better which ones. I suspect that that previous version (which is just called a Cyclone) will work just as well. The Mark II was different in that it was more aerodynamic looking. There are two different lengths of the jockey-wheel cages. You probably want to match whatever you have.
If you find a replacement and want to do the work yourself, be advised that, given the force required to break a derailler, your hanger (the little tab of metal that the derailler fastens to) may be bent and if so will require straightening.
Last, not to be a jerk, but if you snapped a derailler in half while changing a tube, would it be a good idea to take this to the bike shop for a repair?
Last (this time I mean it), can you post a pic of what broke?
Last edited by WizardOfBoz; 04-10-17 at 08:22 AM.