Is it a condition in an Istisna' contract that the supplier be the manufacturer, or is it sufficient for him to undertake the supply through other parties, and does this contract apply to small-sized transactions?

The fundamental principle in a manufacturing contract (Istisna') is that the manufacturer is not required to perform the work himself. Rather, it is permissible for him to purchase what another has manufactured and deliver it, provided it matches the specifications. This is because the contract pertains to the manufactured item itself, not to the manufacturer's act of production. This rule applies unless it is stipulated that he must perform the work himself, or if his individual skill is deemed essential due to his expertise. Istisna' is permissible for all items that meet the conditions of Istisna', whether they are small or large.