Any of the known P6 suppliers should have them,or one of the many Stainless steel exh places.
An S system is large bore,ie, 54mm diameter pipe
Doubt you will find anywhere still selling mild steel items,you may be lucky finding one via ebay.
Last new old stock one I saw sold for £50 buy now very quickly!
I do have a couple of very good s/h items I bought a while ago,the seller thought they were rear boxes,and I didnt ask to see a pic!!! :roll:
One of them I think has had very little use,but could do with a fresh lick of heat resistant paint!!
Where in Dorset are you??