Obivion: vampire dust problem

Hi,

I am doing the mages guild quests. My latest one was to kill some
vampires in a cavern, and afterwards kill some vampire hunters in the
city. Or at least, get rid of them. I do not want to kill them, I don't
like breaking the law. So I talked with one of them and told him the
vampires in the cave were already dead and he wanted proof: vampire
dust. Indeed, all these vampires in the caves had vampire dust when I
looted their bodies. But afterwards, because I was almost over
encumbered, I went to shops to sell found armors and weapons and
amulets, but I also sold my vampire dust. I have been in several stores
all over Tamriel and I really do not remember where I have sold it. It
might even be that I have seen this person and that she has not got het
in her inventory now (I went to several shops to try to buy it back).
So now I cannot go on with my quest.
But maybe, when I kill another vampire, his vampire dust will do too.
So, does anyone know some cave or ruins where I can find vampires to
kill?

Thanks beforehand!