I discover lately this thread.

Militaries furniture belongs to the governement and helmets are part of this. This means that soldiers do not own them.
When a new model arrive, they will provide them in priority to engaged troups, will build stocks "in case of need" till the old versions are all used or destroyed, and then they will sell what remains to the "domaine" where anybody can by them at auction. Usualy sold in big volumes.