I recently did some searching for "pontoon fencing" or "pontoon railing" and I am sure that I ran across the deck edging. Here is one company that sells pontoon parts. Perhaps they will sell the edge extrusion or tell you where you can find it.
http://usapontoon.com/Basic_Pontoon_Kit.php
http://www.pontoonspecialists.com/p-...oon-boats.aspx I think they only carry 102" sections. Perhaps you could fab up some joint covers from alum channel or angle or use a small section the the trim pop riveted over the joint. They also sell "kits" that include hardware and splice plates for the aluminum sections.
Here is a company in Winchester, KY that sells aluminum angle and channel in up to 25' lengths. Shipping might be an issue but give them a call.
https://www.metalsdepot.com/
Doing a search for edge trim I found several other threads and did not see any suppliers. If the boat mfgr. is still in biz maybe give them a call.
Good luck and let us know if you find any.
