For modelers of our Semi-Trailer models in HO scale, the folks at PPW/A-Line have a nice selection of detail parts that can upgrade your trailers. These especially include wheel sets, but also landing gear, spare tire holder, rear doors and more. Here are links to some of their items:
Trailer Dress-Up Kits (Wheel Sets)
Trailer Kits and Parts (Landing Gear, Flaps, Side Skirts)
Miscellaneous Vehicle Parts (Spare Tire Rack, Document Boxes)