Melvyn Rutter seems to do all the parts but you may need to ring/email for the full system. Librands also have some stainless options. However, both will be much more expensive than having one made up at a exhaust specialist as Arwyn suggests.

I built my own out of stainless ebay bits - mine is intentionally quite loud but could have gone for a larger silencer if needed. Cost about £80 IIRC

"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10