What about using Summoner's Pact?
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What about Urza's Guilt?
Only problem with the Cho-manno + pariah fix is that you're depending on two cards. I run a black / white version of this deck and I use Zuran Orb in it. Also, waiting to play Cho-Manno and Pariah after you've played Karma (11 mana total) with no mana acceleration could take a while. Zuran Orb is free... and an artifact, so you can use Enlightened Tutor to get it if you need to.
Very nice deck. Have you considered Plague Wind? Maybe some Culling the Weak for mana acceleration too.
What about Buried Alive or Entomb?