Pretty much any fly shop can help you. There is a wheel that is (not sure if they're still doing it) made in Indianola, WA, that is made to specifically fit your boat. I've seen lots of homemade wheel options, but this one is pretty slick. Fat, inflatable tire that rolls easily through sand, the wheel folds up out of the way when fishing, etc.
Also, there is extensive discussion about do-it-yourself pontoon boat wheel options in the Watercraft and the Stillwater sections of this website.