Can someone introduce me where to buy cheap and good air humidifier ?

Qoo10 has really cheap ones but not too sure about the quality. It's always hit or miss with Qoo10, sometimes you get a good deal, sometimes it can be quite dodgy. Hence, I suggest you try reputable places like Harvey Norman -- they have great sales sometimes, and there's warranty when you buy at these places. Another place you can try is Action City.