Preferential Supplier Policy

I manage a medium sized business with various departments which buy in external services off external suppliers for training, hotels , travel etc.
To manage cost , I would like to introduce a preferred supplier list. I need help in the formulation of a policy document to launch this.
The policy document should include :
- The Policy
- The preferred supplier lists.
- The authorisation processes.
The authorisation process will be department Head to pass to Executive director to sign off.
The check and Balance case procedure will be unless Executive director to signature is on the document the spend request will be rejected by Finance manager.
In instances where the department head use a supplier off the preferred supplier list, a business case is made in writing.