At home I've been using 4 German Venta brand humidifiers, bought them about 5 years ago and I have forgotten where, but I think they are still available in Beijing. If I find out where I will post this info. This kind works by passive evaporation of air passing over a drum immersed in water. The steam kind (like boiling a kettle) that Liora mentions are also good, but as she says not easy to get here. Tap water is fine in both these kinds since the salts stay in the reservoir.

I don't know about medical grade De-ionized water availability, but that sounds expensive! Water delivered in plastic drums is ok I think if it is distilled since this gets rid of the salts, but watch out for mineralized waters that have had the salts put back in. With the Nestle brand water this is the default option these days, but you can still get the pure distilled variety if you ask for it. There may be good reasons for drinking the mineralized versions of course, but they are not good for ultrasonic humidifiers. The Watsons water I have at home still seems to be the pure distilled variety.

Another option is to install a Reverse Osmosis (ROI) water purifier under your sink since this kind gets rid of water hardness. There are several brands available in the home appliance stores. Ion-exchange softeners don't help with the salt content because they just swap one kind of salt (calcium water hardness) for another (common salt). Regular water filters, water treatment jugs and so on also do not remove hardness.