MTH has a number of the cars, I have the REA 20-94211 version.
Harry Henning did the conversion of these two units with his using Atlas bodies, When I was down at York, I picked up a couple of old Atlas scale cars, a reefer and a cattle car with the same conversion in mind. You just make some spacers along the sides to secure it to the car frame. The only tricky part is getting the fill capability for the hot box reefer, the cattle car doesn't need anything special, all the controls are on the frame. Of course, the hot box reefer is converted to TMCC as well.
