I would suggest a local marina, e-bay or craigs list. I have purchased 2 pontoons in the last 12 years and in both cases I was looking for a nice, late model used pontoon. Guess what, both times, I ended up buying new. Most people tend to hand onto their pontoons and the used ones I found were either completely trashed (no maintenance, left outside all winter...) or not much cheaper than a new one. Some marinas have nice rental fleets they sell off every couple of years but those will be high hour, somewhat abused units. Never purchase a used boat with out a water test and having it looked over by a qualified tech. Also, crawl all over it with a fine tooth comb and look for signs of damage and abuse. Open up all the seats and storage compartments, look under the helm, make sure all the switches work and so on. Good luck with the search.
