Tmmar. I don't know if you read my recent posts regarding my RM1350 fridge
(which actually comes from the factory with two axial fans already installed) but I installed a 107CFM, 12V fan into the vent on the roof. I taped into the existing fan power lines after the factory installed thermal switch. This way the fan comes on only when needed with the factory fans. This could however be hard to do without pulling the fridge out.
For a source of 12 volts you can just insert your wires into the supply block on the back of the fridge where the 12 volt wires for the fridge come in. They are readily accessible from the outside access plate. You will more than likely have a white wire A(+12v) and a green wire (-12V). Ours also has a blue wire that is the alternator line from the engine that actuates the automatic door lock. You could either wire in a switch on the line to turn it on and off manually or get a thermal switch like the ones that Dometic uses at an RV supply store and wire it inline to turn the fan on only when needed.
I bought my fan through WW Grainger. Be sure to select a fan that can be mounted horizontally. They stock all different sizes and CFM ratings. I would say that anything under about 60CFM isn't gonna do you much good. I believe that Radio Shack sells axial fans that are about 30cfm for around $11, but again the horizontal mount fans will cost more.