Bachmann's idea of "a part" is an entire sub assembly at half the retail price of an entire engine and, even if you're willing to pay the price, you might as well buy a whole other engine and then part our what you don't need.
On the other end of the spectrum you have Accucraft. I just bought a bunch of their cars, second hand, and discovered that about a dozen of them had the early trucks that don't roll well. They need the "L" brace modification that was added later. I called them up, they asked how many I needed and they mailed the parts to me free of charge, even though they knew I didn't purchase the cars from them.