New, or used? Any specific brand?
Otherwise, just the usual.
eBay, Craigslist, swap meet.
Or you can make your own. A few years ago there were tons of the ones that came off of the M-1008s that the Defense Department sold off. Many guys grabbed up those trucks for the axles and scrapped the rest. I'm sure tons of good or at least usable sheet metal went to the shredder because the rock crawler people didn't care to try to sell the carcasses of the M-series military trucks.