Suppliers are your main source of items, so it is imperative that you get as many as you can to get more items and keep crafting all the time.
Each time you buy a Supplier, buying another one will cost more than before. However, if you upgrade the Supplier, the cost of getting a brand new one goes down again!
Ensure you always upgrade your previous suppliers before getting new ones to maximize your Supplier cost-efficiency!
Upgrading suppliers makes them give more items, but they take longer to give the rewards. It is important to keep a balance between low-level suppliers and high level ones.