I have a 3" mongoose cat back, after £150 + postage, good condition, would be like knew with a polish, used for around 18 months. Let me know if you're interested and I'll sort some pics!
if you are going to stay NA, can i recommend you find a free flow header, then what ever the collection pipes final size, run with that diameter for the rest of your exhaust, that way you will get a performance gain and the correct size to maintain exhaust flow