From the Place Order screen, begin an order with the Supplier you would like to add a sales rep to.

From the Delivery Date screen choose Edit Supplier.

Under the contact for your supplier, you can choose which Locations are associated with each contact.

To remove a location, select the X next to the location.

Select + Add another email to add another Sales Rep's email address and select the location they are meant to receive orders from.

Save when finished.