Anything you have laying around in the scrap box can work. Broken couplers or pull the plastic wheels off an axle if you replace your wheelsets with metal ones. Broken gears from an engine you repaired. Some building window castings that got messed up, include a broken piece of clear styrene to simulate broken glass. Vehicles you no longer like or no longer fit your plans can have a fender or two cut off and left abandoned after a good rust and weathering job. The box bodies from an old truck kit make a neat storaage shed next to a scrap pile. A piece of 1/4 x 5/16 rect styrene can be an old refrigerator with 2 thin strip styrene doors cemented on it ( HO size ).
Once you start digging around you'll come up with ideas.