First
you should understand what is that Order represent, something the Supplier buy or sell/provide.
Not knowing your use case how Supplier or Order is used, it's hard to determine the exact relationship
Every Supplier
<fill in the blank> many Orders.
What's the purchasing department doing there in the domain model? Or did you put it there? The purchasing department should be an actor in the use case diagram and actors are rarely in the domain model unless it's really important.