you can order online at anyplace that sells pontoon boat accessories or you can try some of your local fly shops (they seem to have more pontoon boats and accessories for sale), or try Wholesale sports (formerly Sportsman's Warehouse).
I got mine at Sport'smans Warehouse. And have seen them online at The Fly Shop in Welches Oregon, Cabellas, Bass Pro, and other places.
make sure you get the larger rod mount... I purchased the smaller diameter without thinking and had to take it back.
Wholesale Wearhouse has a fly rod holder they sell (I think its branded as a float tube mount), but the mounting brack it comes with has 24" straps that you can use around the pontoon if you dont want to put it on the frame or your foot rest.