Sounds like you are talking about military surplus, and those things are getting harder to find than beer on day 14 of a 15 day Grand trip.
But, they are out there. If I wanted one, I'd start with military surplus stores and outlets cause they get them occasionally, or even military surplus right from the government.
Then I'd go to the Grand Canyon outfitters, but I suspect they would rather loose permit days than give up even their warehouse queen boats. Even if they don't float anymore, they are good for scrap and what not.
Expect to pay a lot, and you damn sure better know your repair skills and what you want to do with it before you latch onto it.
Those things are like cats; you simply cannot abandon one you have acquired.