Most watch companies will buy some pieces like this balance spring off the shelves of subcontractors.
BUT...if you want something done well, and custom made to provide the right torque and counter weight...and to be GOOD.
These IWC springs for the Caliber 5000, and NOT for all IWC watches are:
Hand made and bent
Tiny painful work
In other words...they will last forever, not rust, and will be EXPENSIVE.
And for a GOOD reason.
There were a large number of skilled Indian and Thai workers at the artisan levels...and a large number of local Swiss and germans at the executive levels...marketing people....many can tell the difference between a watch and a pocket watch, if you remove the cases.