How to automatically create purchase order from internally sourced requisition

Our current setup is to create purchase orders for all expense requisitions. However we'd prefer to source expense reqs internally for first depending on the deliver to location, and then automatically create a purchase order if stock isn't available. The flow is demonstrated in the example below.

Ex. Departments A, B, & C all created expense requisitions. Depts A & B should source internally while Dept C should only source externally.

• Dept C - expense requisition sources externally only and automatically creates purchase order.
• Dept A - expense requisition is for an item with stock available. The system automatically creates transfer order and fulfills order.