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
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