where to buy?

Try Spit Cycles in Mosman. The last time I did it on my GT they sourced one easily and quickly (took them three days fix and replace the parts, including a full rear wheel rebuild from where the now destroyed derailleur went through the wheel...).

I have never sh@# myself so much after it gave way in the river at the oaks at about 45 kmph destroying the rear wheel and sending me over the bars at serious speed (thank god the water was high enough to break my fall) - and my wife though I had done it on purpose to save having to ride up out of the gorge...