Hey Steve. No big deal. I honestly never saw the truck in question for separate sale at a show but maybe a search of the Lionel parts catalog (on line) may help. Look for part number of an articulated set (like your NW) and see if they list them for sale. There is usually an exploded parts diagram with part numbers and a list of available parts.
Hope that helps.
If not having any luck, just do what I do. I run the double articulated cars together and the singles just get added like a regular freight car in the front or back.