Mcmaster Carr
You might try McMaster Carr for the pieces. They have stainless steel pipe sections (the same sizes as the galvanized you can get at Home Depot/Lowes).
It *can* be done without going totally custom.
Moyer and McMaster can help.
ps....here's my secret, my wife and I surveyed Sirroco when we were debating repairing Ariel Spirit after H/C Isabel or selling and buying new. Item number 1 on the surveyor report was the tube-in-tube exhaust that I imagine you are replacing. Good Manuever getting rid of that thing! In fact, I almost pushed the button to order all the parts and pieces from McMaster when we decided to not buy Sirroco and instead repair Ariel Spirit.
McMaster seems to have lots of good boat repair stuff.
For example, tyvek coveralls-about $5, a far cry from the highway robbery that Worst Marine charges.
-Solid fiberglass panels in sizes up to 4'x8', and thicknesses up to aroung 1" solid fiberglass. The possibilities are numerous, including mounting pads and backing plates (without getting into drilling through metal).
-Fasteners and fasteners.
-Those cool little pins you use to attach sound insulation to the engine compartment, for about 1/100th the cost in a marine out let, for the same thing.
you get the idea. But for more ideas, here is a guy doing a major refit of a C&C 38 Landfall. I know I know, heresy mentioning another boat on this forum, but frankly, boat repair is boat repair is boat repair. He mentions McMaster, and typically includes the catalog number for the stuff he uses!
http://www.wbryant.com/StellaBoat/projlist.htm
-km
aka, "sell out"
S/V Beyond the Sea
C&C 35 mkIII