When searching for information about ancient pole weapons, arrows and javelins, the information seems to focus on the heads of these weapons (the metalworking process, the design of the blade, etc.).

I can't seem to find much information about how the shafts were constructed (or obtained). For example, the Macedonian Sarissa or Kontos, which were very long (and apparently sometimes consisted of connecting sub-shafts).

In accounts of the massacre at Carrhae, we are told that the Parthian horse-archers continuously restocked their supply of arrows at their baggage train, enough to fire continuously for (presumably) hours. How were the shafts for such an enormous quantity of arrows obtained in such terrain?

We know that the Romans designed their pila to foul after use, to prevent ad-hoc re-use on the battlefield by the enemy. Was it likely that arrows/spear shafts (as well as the harder-to-manufacture heads) were retrieved after battle, to reduce the time it took to rearm similarly equipped troops in the field?