1) 4 wheel trucks W/ magnetic or coil coupler, W/ center roller pickup, C clipped center post (Not riveted to rolling stock) To illuminate some old favorite cabooses.
Deeper yet inside the magnetically uncoupled trucks, 2) the flat leaf spring. (usually copper or bronze color) and 3) the rivet that holds the leaf spring/bottom plate to the truck assembly. Repairing/cleaning about 4 or 5 dozen of these trucks.
Definitely looking for new or generic equivalents to the post war rivets.
Don’t know any P/N’s yet, as I’m missing part lists in my collection from mid 40’s through the 1970’s.
I agree with Ben on finding junkers. You could easily spend much more on individual parts. Another great source is George Teabolt. He sells complete truck assemblies and the individual parts you describe (rivets, rollers,springs, etc). You really should have a service manual for part numbers since there is a variety. However, he is pretty sharp on the phone when you describe what you need.
Carl Scire sells the Brakeman’s riveting tools. He has a line of riveters and other misc tools especially designed for repairing Lionel truck assemblies.
Ben and and John are right, look for junk cars or stop by a store that has been around for awhile. I needed a coupler head (~$2) and found a complete truck with coupler for the same price at a train store.