Might be struggling for a new Mazda 3 2.0 exhaust on a budget. It looks like it was just one big piece from the manifold back, installed in a oner then the rear suspension installed after that. It’ll be costly because of all the work required to change it.
You can get MPS exhausts cheap enough, but they are a bigger diameter. You may be able to knock up a reducer somehow.
If you don’t want some boy racer bean can thing, which I’m guessing a lot of the ones you can buy over here are, I’d be look at a Racing Beat exhaust from the States. About £350 + import duty, so maybe £450 total? Looks like it does away with faffing about with the rear suspension too, as it comes in bits which clamp together. Despite the name, they are very refined indeed. I had one on my old RX7, which was originally very loud and obnoxious to the point of being embarrassing to drive. One Racing Beat later and it was as quiet as the wife’s Ford Ka.