I've found a good bit of stuff at gunshows, from beatup 1911s to parts to tools and books. Tucson and Phoenix both have sizable gunshows. The little show up at Kingman in February (dunno if there are other dates) often has lots of parts and pieces; more hunting guns and less para-military.
The two main aspects of "fit" are the barrel-link and the bushing. The longer the link, the tighter the lockup of the barrel to the slide. Of course, you can go too far with this, and it is indeed a fit-and-try deal. Getting the bushing right is easy although somewhat time consuming.
As for the fit of the slide to the frame, that's a mix of careful hammer-tapping and then clearancing with valve-grinding compound. Again, caution; it's a bit time-consuming but no big deal.
I polish the surface of the feed ramp and the throat of the chamber, and the top of the hood and chamber. This is jeweler's-rouge polishing of the surface, not removal of metal. It reduces friction for the entry of a cartridge.
Hallock's book shows the angles and depth for stoning the hammer and sear. Lotsa caution, plus a feeler gauge and a 16X lens makes this part of the trigger job work...
Another deal to take the "foreplay" out of a trigger is to lay brazing material across the back flat of the trigger. (I made a "heat sink" by grinding the head of a large bolt, to fit snugly inside the trigger; then mount it in a vise.) There is then the tedium of filing it down to where it looks like it might fit, and then assembling and testing. Eventually, you can get it to where there might be some 10 or fifteen thousandths of an inch of takeup before the trigger pushes the sear.
And now you know as much as I do.
, Art