Officer Klemis:
You can get Soul Stone Dust from any monster in the Seed of Annihilation area. Bring me 5 and I'll give you 1 Soul Stone Fragment.
But it's such a little thing that you probably want to bring at least 50 at a time. I need to make the exchange worth my while too, you know. And don't bother bringing more than 500, because I don't have a magic bag of endless stuff to give you.