I would expect them to be available via Suzuki dealers - try Levois of Colchester - they give us 10% discount
https://www.suzuki-shop.co.uk/.
I doubt any aftermarket supplier will have Cappo exhausts - not enough demand.
The other way is direct from Japan e.g.
Amayama, and search by part number (from the parts catalog - click Manuals).
My '93 Cappo is still on its original exhaust.
The cheapest option is probably a lifetime SS system. As they are made up to order, you can specify a well muffled box. They will probably be surprised, as
most of their customers want the opposite
I have a SS system on my Grand Vitara - it was a bit noisy at first, but quietened down as it sooted up. New MS systems are also, at first, noisier than those they replace, for the same reason.