i don't think the brand matters too much? you can go to these electronic depots on trang tien or tran hung dao. they blast loud electronic music so it should be easier to find.
if you google "how to buy a dehumidifier" you can get some good results on capacity: http://www.lowes.com/cd_Dehumidifer+Buying+Guide_174021230_
also if you search the tnh forums, some good hits may also come up
Very difficult to find a descent one at the moment. The big electronic shops on Hai Ba Trung doesnt have them. I checked a couple of shops including Media Mart and nearby shops and no one had them. I was then directed to a shop at 12 Trang Thi, Hoan Kiem. They had 5 different types and brands ranging from VND3,600,000.00 to the most expensive one at VND12,000.00. I bought a Japanese named one (made in Vietnam of course) and its doing the job.