In replying to this thread on the Unknown Armies Fan Club, I inadvertently wrote up a small collection of merchants dealing in eldritch wares. Roll 1d8, pick one, or shove all of them into the same eldritch market:
1) A grotesque old hag who deals in children's nightmares. Becomes very interesting when the player characters realize she has access to a nightmare that contains valuable information — maybe the location of a hidden treasure, enough information to solve a murder, or a prophetic dream.
2) A maggot with a human face that deals in luck. If your fantasy game has a luck stat, use that; otherwise, just say you can trade bad rolls for good. (Or good rolls for bad.) Your choice whether the maggot-man wants your bad luck or good.
3) A necromancer dealing in ghosts. Who doesn't want their own invisible spy and servant? Of course, you probably have to make another ghost for him to summon in exchange...
4) A forgotten god that's willing to deal if you offer it worship. This offer has to be genuine — you're not going to fool it if your faith is lacking.
5) A humanoid creature — too fat, too leering, too many joints on its fingers — that deals in rare opium. Use tables from Hubris or Yoon-Suin for inspiration, or if you don't have access to those books, let this magic opium operate like magic potions. (Or, alternately, it's just really good opium.)
6) An ancient vampire that's willing to trade secrets for rare vintages of blood. Oh yeah, it knows exactly when the Dark Lord's army is going to strike, but now you've got to track down an eight hundred year-old elf virgin who's been suffering from paranoia for a hundred years and somehow harvest its blood.
7) A talking, hyper-intelligent crow. It potentially has something valuable among the baubles it hoards, but you can only take its hoard if you can put it "back to sleep" as it were and return its intelligence to normal crow levels.
8) A cursed man who can pass on his curse if someone else willingly accepts it. He has what you need, if you're willing to accept his curse.