why do you need one? got rid of your cat and have a check engine light?
some aftermarket exhausts come with 02 simulators i think.
SERs have two cats, and i've heard of ser owners relocating their 02 sensor downwind from the second cat when they get a new header.
if your running w/o any catalytic converter, then as far as i know, you'll just have to deal with the check engine light. i'm not too knowledgable about this stuff tho, sorry.