Here are the pages showing derailleur hangers in the Quality catalog.
Disc brake pads are a less detailed jigsaw puzzle, but still important to get right.
The bike in front of me takes a Wheels Manufacturing number 43. We have plastic tubs full of Wheels Manufacturing number 27, accumulated from Fujis that came with two in the box. The 27 looks somewhat like the 43, but not close enough to tempt improvisation.
Because most hangers are alloy, they often snap when you try to coax them back from what looks like a minor deviation. They're meant to fail to save the frame and possibly the derailleur. A wise off-road rider should buy one or two spares ahead of time. I can't imagine too many shops bother to keep every style fully stocked.
Yesterday and the day before I spent bringing a 1991 Specialized Hardrock back from the dead.
Here's what I found inside the left shifter:
That's some kind of old bug nest.
This thing was profoundly cruddy. It went well with everything else that was rusted to a lump.
The derailleurs wouldn't even move. A day and a half of penetrating oil actually loosened them up. Hard to believe the bike functions, but it does. I was about ready to declare it dead when eyelids fluttered, metaphorically speaking.