Hi Phillip,
In general for any factory-built aircraft, your best bet for non-Rotax replacement components is usually the aircraft manufacturer or their local agent. Your profile picture shows what appears to be an Evektor Sportstar or Harmony - if that is in fact your aircraft, the Illustrated Parts Catalogue is available as a free download on Evektor's website. If you find the part you need in the catalogue, you should be able to order a new part through the manufacturer or their Australian dealer.
If you wish to install an aftermarket exhaust system, it is important to first establish for certain whether your aircraft is actually type certified or an LSA (S-LSA for anyone reading this from the US) because your options for having aftermarket modifications approved are significantly different between the two. Evektor produces both type certified and LSA versions of their aircraft.