If I were to guess, I would say the best place to get a Soft Sister inspired look would be places such as KOKOKim, Candy Stripper and Syrup. These shops would certainly be a good place to start.
And here are a few Taobao stores that seem to pop up a lot whilst I was searching Taobao. I personally really like Bobon21-
Firefly Girl- http://yhsn33.taobao.com/?spm=2013.1.0.0.jgtf8C
Whilst I am unsure whether I would wear these clothes myself, there are definitely some very cute items that pop up when using the Soft Sister search term. It is worth a quick browse to see what you can find. Soft Sister may be more of a search term rather than an actual fashion, but it brings up some interesting results!