Expedia.com allows you to search for the cheapest flights across all types of airlines.
Better do the search using Expedia. It will tell you all the avilable options. After that you may book directly using the original airlines. One problem with Expedia is that it cannot pay for children under the age of 18. Make sure you book from Kota Kinabalu. Do not try to book from KL first. I found out that KK - Istanbul is the same price as KL - Istanbul, using Expedia and Qatar Airways.
I ended up booking using Qatar Airways from KK. Qatar Airways has joint venture with MAS for the flight to Istanbul. At RM2000 is almost the same price as to Seoul which costs RM1500 via KL. Direct flights to Seoul cost RM2300.