Check out
http://www.racingjunk.com. There are a lot of units there and a very active classified section. I found my unit at
http://www.truckpaper.com it is almost entirely commercial trucks, so my find was an anomally.
As far as I can tell there aren't any comprehensive reviews of the units available. My advice is to read everything that you can get your hands on. This site and the escapees site has a lot of good information.
http://www.escapees.com