One of the most difficult situations with the watch collecting and repair hobby is when you discover you have a broken part. When this occurs, I usually get a sinking feeling in my gut that the watch is worthless. There is a chance that your watch can still be saved.
Sometimes the replacement of a broken part can be difficult and require great skill to get it right. You may want to call on a watchmaker when it comes to balance staffs, or if one of the gears is broken. These jobs are tricky. If you don't have a watchmaker see WATCH REPAIR SHOPS for info on finding one.
For the beginner, some easy parts to replace on your own are: crystals, hands, mainsprings, detents, clicks, click springs, cannon pinion, winding gears, crowns, stems etc.
Even when it comes to broken balance staffs, I would try to find a "donor" movement by searching on ebay. You just need to locate the CALIBER on the watch movement...
Here's an example: You have a watch with a broken mainspring. The following words appear in various places on the movement:
ELGIN 670 U.S.A. = Manufacturer and Caliber/Model
21 Jewels = # of jewels
ADJ'D = Adjusted
R 382566 = Serial Number
For example if you perform title and description search for "ELGIN 670", you will find some movements for sale that you can salvage parts from.