Ceramic coating but a lot to spend onstock cans.
I have a perfect fix for you....... find a set of these
Big Boss model Jardines with performance headers.
These have been re-furbished in black ceramic coating inside and out.
As noted in a recent thread there are a few versions, single and dual pipe in chrome or black with different O2 bung location on pre-ABS models like yours to the later ABS Roadsters. I think they did a RIII Touring version too.
Otherwise find a set of Black factory Roadster dual pipes TORs Twin Silencer Kit, Black - Roadster Special Edition A9608144. I sold a few new sets on the forum a few years ago and saw one in Germany recently but it has now gone.
Easy to modify hangers to fit and they fit stock headers but you will need a Roadster collector cat free box . Likely about 20 will be offered for free if you start a wanted thread in the Classifieds.
I doubt these dealers will have a set but you never know.
Genuine Triumph Part # A9608144 - Silencer, Twin, Black, Kit. Edition, KITS, Motorcycle, SILENCERS. Fits Rocket III. Ships from Cross Country Triumph, Metuchen NJ
triumphparts.crosscountrycycle.net