The Protec 21 was (is?) made by SACS, an Italian company. You could try contacting
RYB Marine Sales Ltd
Phone: 01753 851717
Who were importing SACS RIBs last time I heard, or you could call Paul Lemmer who was importing the Protec 21 a year or two ago. His number is 01737 644307
There have been a number of other smallish cabin RIBs made, but most of them seem to be one-offs.
Production cabin RIBs tend to be rather larger, around the 9-10 metre mark. Manufacturers include Ribtec, South Coast RIBs (Scorpion), Ribcraft, Delta, Cowes Marine. Details in the suppliers section.
Hope this helps.