The Sisters may be found anywhere from street corners, where they offer to tell ones future for a few pennies, to those who appear to be working for the service of a lord. It is said that they go where they will, when they will they work for who they choose, not for who chooses them.
You receive a vision of a rooftop "somewhere close by", where a rare moss grows; in the vision, you know the moss is suitable for greatly enhancing the effects of certain types of potions. Now go and find that rooftop. Don't fall off. (Of course, rooftop runners *will* be taken for miscreants by the local City Watch....)