You can get an O2 sensor at any auto parts store. I think in 04 they made them with 4 sensors, since there's 2 cats (2 before, 2 after). Someone will chime in to either support what I've said or refute it. My parts book says I should have 4 with 2 cats, but in reality (as in looking under the truck) I have only 2 with 1 cat. I recommend getting up in there and visually verifying it yourself before you buy anything. Hell, if you pull one down you can even get a part number off it.