I know that it is surprising that even here in a Muslim country you can't find appropriate clothes!! I guess it's because most women wear ababys to cover themsleves. However the store Max does have some nice long skirts and longer tops which are well priced. There are two Max stores in Doha. One is in CityCentre Mall, but it is quite small, especially the section for women's clothes. The other is near the family food centre and is very big. There must be a lot of other options that other people know about..........
There is a facebook page of a woman selling Islamic clothing here in Doha. She has a small collection and the other stuff has to be ordered from overseas, but then is delivered to your door. Check it out: https://www.facebook.com/qatarmuslimfashion
hope that helps, IA