Something of that scale I would buy at a motorcycle/atv junkyard. Normally my parts come from ebay or biker bandit. I don't know of a place that sales rear end parts for an atv. As Lloyd recommended, I would look for an entire assembly. I can set up rear ends, but would not attempt an atv due to lack of experience and new parts. Would be a double pain in the butt to try to get used parts wore a certain way to mesh with what you have that wore a different way. As well as a place to find the bearings, shims and seals.
BTW did a quick search on ebay motors and there are a few complete assemblies listed. Good luck.