I've looked everywhere for the after catalytic converter O2 sensor for my 2k maxima because I thought they were very expensive. I tried local aftermarket part suppliers and have been quoted anywhere between $250 and $450 Canadian. I tried online suppliers and the best they could do was $80US ($115 Canadian). Today I stopped in at the dealer just to ask, and I bought the OEM part for $50 Canadian ($35US). By far the best deal around. I don't know if they take mail orders, but the dealer is "Dermac Nissan" in Brampton, Ontario.

http://dermacnissan.com/parts.html