Speed-Shops are a good source for Stainless flex.
Navistar (International) uses 2-1/2" flex on some applications, so a Truck Parts store or Dealership like International or Freightliner would be a goo place to check.
Have you considered takeing the boat to a muffler shop and haveing pipes bent?
Try looking for a place that offers custom exhausts, NOT some place like Midas, I would bet they could bend up a super nice system for less money and weight than using flex pipe. If there are any salvage or scrap metal places near you try and find some stainless steel pipe: food processing plants never re-use it, the machines for bending muffler tubeing will bend the stuff just as easily and it'll last forever! Not to mention that 2-1/2" is a common size. If flexion and vibration is a concern you might get lucky and find a braded stainless section, or if nothing else use a short piece of flex to join them. Never seen this done on an AB, but have seen it done on LOTS of trucks. YMMV