I work part-time for Paul at Motormech / RoarPower - I've known Paul for ten+ years and he's always worked on all my cars. My 480bhp Fiat Coupe runs a 3" RoarPower exhaust that has now done 150,000+ miles, so the quality really isn't in question
If all you're after is a cheap exhaust system, then any of Scottyf's links above could be suitable.
However, Longlife provide a transferable lifetime warranty on all their exhausts, so quite simply, regardless of who owns the car, a Longlife exhaust is the last exhaust a car will ever need.
Additionally, because all Longlife systems are custom-built onto the car, the owner gets to specify the look and sound - Paul will then build to your specs.
With Motormech in the workshop next door, we get to build lots of exhausts for Alfas and Fiats - its always nice when we get a V6 Alfa or a 5-pot Fiat in, because there's always a lovely sound just waiting to be released. Without knocking the GM-derived V6s, there simply isn't anything this side of a Ferrari or Lambo that sounds like a Busso lump with a good exhaust
Lets see what the feedback is when we've done this one, but we would be happy to set up a group-buy for AlfaOwner members