try testpipes.com. this is where i got my testpipe and its pretty nice...fit perfectly..etc..and the guy i talked to when ordering said they can include a place for the o2 sensor...if the car is obd2. not sure about check engine lights in that case...as i've never had an obd2 car...
also, high flow cats in my opinion are a gimic. if you're really worried about damaging .0000001% of the atmosphere and absolutely need one just get anything thats 2.25 inches in diameter. the difference in restriction and horsepower gains will be very small and probably not noticeable.
just don't spend 160 for a cat...its such a waste of money.