Rimmers do mail order for replacement exhausts if you can DIY, or get your local 'garage' to fit it for you. However, many people are fitting 'Longlife' stainless steel exhausts. Longlife is a nationwide franchise; if you do a search on 'Google' you will find out about their product and your nearest dealer. Although more expensive to start with, it is a once-only purchase, with a lifetime transferable guarantee.
If you go for the full SS system, I would suggest you keep the 'Cat', because I found the exhaust was a bit rorty without it, so I had it re-fitted; other owners have removed the 'Cat' and are happy with it. If you want a quiet exhaust, which most Rover owners do, ask for the quote to be for the 'very quiet version'. It will be made to your requirements.