There are about 35 different atv and atc that use the same rear caliper. I bought a mojave rear caliper on ebay for my 300ex. All you have to do to make it fit is swap the parking brake portion of the caliper. I had a list if I can find it I will post it up.
Ebay for shure. Look for a not so popular model like the mojave. Normally they run 60-120 (ebay). I got mine on ebay for 28 with shiping. Keep in mind you will have to swap the p-brake parts if you get the same caliper off a yamaha or some other brand. Or just use a block off plate.
GDH does sell top notch used parts, nice and clean.