I just replaced my manifold with this:
http://www.amazon.com/dp/B000C8P8M0/ref=pe_175190_21431760_M3T1_ST1_dp_1
It was cheap. I had to slightly modify it to get it to fit (hacksaw off a very small bit of the edge where it meets the head, if you'd like me to post pictures, I can.)
It seem to have had decent ratings on Amazon; that's why I purchased it. I've only had it in for a few days, seems to be working well so far. If you do decide to go this route, I can offer my assistance with anything needed.
My initial problem was the same -- cracked manifold, fault code on upstream O2 sensor. O2 sensor needed replaced. I had already purchased a bosch replacement sensor from autozone for $65. I pulled a downstream from the local junkyard for $12.70 just for the hell of it, and saved my old downstream as a backup.
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