Requisition Approval including Department head

We have a requirement to for Requisition approval to have Direct Manager and Department head for Purchase Requisition approval.

Direct manager can be handled with Supervisor hierarchy, How ever we need the best way for Department head approval as we have lots of departments.

For department head we tried to use Cost Center Manager approval but for Fixed assets and Inventory cost center will be default so we will have a problem in this case, So is there a best practice to easily capture department from requester and get the manager ?