Craigslist and forums like this. Before buying from a dealership look up the google and yelp reviews to see what their reputation is. Other peoples reviews can save you a lot of time and money.
I would run like hell from anything being sold by Mastadon rather you can register it or not. They specialize in G-Wagons that could not be sold elsewhere because of condition. G-Wagons are great trucks but when not taken care of properly they can become a money pit.
