The easiest way of finding health insurance is to go through a broker. They will discuss your needs and provide the best options. You can access a broker through this website but there are many others: www.insuranceinAsia.com
NOTE: Most expats use Bao Viet and Liberty. Bao Viet is cheaper and flexible in terms of your needs. Liberty will allow you to pay half yearly. If you can afford nothing else, get Global Rescue. At the very least it will get you home in the case of an emergency. If you want a cheaper policy, think about raising your excess.