The mechaber rules like the Rashba that hair is considered part of the body regarding the issur of having hanaa from a corpse (the rambam rules it is not but the sa follows the strict opinion of rashba). Based on this psak in shulchan aruch I highly doubt frum sheitel machers would use hair from cadavers (hopefully someone has asked after all these years!!!). It is true though that cadavers are used to produce wigs in the non jewish world.
