I am looking for a brush for ragged and torn cloth edges.
I don’t think there is a brush for that. There are several ways to do it
The easy way are alphas (scroll down to find the guide):
http://www.zbrushcentral.com/printthread.php?pp=40&t=178538&page=5
The difficult way with geometry nanomesh, but I guess there are more ways. These are 3 links to what is basically the same method:
http://www.zbrushcentral.com/showthread.php?177722-Torn-Cloth-Tutorial