I know very little about Hellenistic warfare, but it seems no one is answering this, so for what it's worth:

Philip II of Macedon originally built his army on a very different principle than the armies of ordinary Greek city-states. Where the poleis relied on a large section of reasonably well-to-do citizens to equip themselves as heavy infantry and flesh out the battle line, Philip's homeland mainly boasted an elite of large landowners and a large population of poor, disenfranchised peasants and herdsmen from remote parts of northern Greece. The landowners fought as cavalry; in order to increase the efficiency and numbers of his army, Philip decided to equip the rest of the levy as heavy infantry and to train them at his own expense. The capture of the silver and gold mines at Amphipolis allowed him to maintain standing forces at an unprecedented scale.

Given this background, scholars tend to assume that Philip himself paid for the equipment of his pikemen when he reformed the Macedonian army. Unfortunately, the reforms are extremely poorly attested in the sources, and most of what we claim to know about them is conjecture.

Since the states of the Diadochoi were similarly founded on a large centralised system of state finance and a large professional or partly professional army, it is not unrealistic to assume that Philip's method of equipping soldiers at state expense remained the norm. This would also allow for the introduction of uniform equipment, which would explain references to units called argyraspides (Silver Shields), leukaspides (White Shields) and the like. As the kingdoms of the Diadochoi solidified, many of their soldiers were settled in military colonies which exchanged service obligations for land; in such settlements, equipment may well have been passed down from father to son within soldier-settler families.

How Alexander himself managed to keep his army equipped, I do not know. It is possible that he simply employed local weapons manufacturers. Alternatively, he may have increasingly experimented with local troop types precisely because his original units and their equipment were harder and harder to maintain. On the other hand, he was in frequent communication with his homeland, sending money and asking for reinforcements; it is not unlikely that spare equipment was also sent along.