mm, my parents went to their office to buy directly from the agents. I think this is the easiest way. or you can go to their official website: http://www.aigdirect.com.cn/travel1_3664_532308.html and click '"立即投保“, but to buy it online is difficult if you don't have 支付宝 or a Chinese credit card to bill to. trust me, the easiest and the most reliable way is for your parents to buy it in person if there's an office in your city.
my friend also has bought the insurance from this website: http://wwwhuaren.us.cn/ there are other websites selling this insurance.
Thanks for sharing!