Where to buy a 125cc engine
You can be able to purchase that from online stores like Alibaba.com. They do have promotional engines of 125 cc therefore the prices are affordable as well as their quality. They do have a variety of such products that you can choose from according to your desire. Check them out on http://www.alibaba.com/promotion/promotion_motorcycle-engine-125cc-promotion-list.html for more info.
You can buy one on E-Bay, Craigslist, anywhere you think there might be one, really.