Student or Learner
If I would say that businesses are filling new orders out of existing inventory, is it also correct to say that "businesses are filling new order with existing inventory"? or fill new orders from existing inventory? if not, what is the difference?
You need to keep the present continuous - it's what they are doing now.
You can say with or from, however, without changing the meaning. The key is that although they are getting and filling orders, it's not enough yet that they are ordering/making new products themselves yet. Once THAT happens, the economy will start moving more quickly.