Most organizations have a Commercial department that is responsible for negotiating with third parties to obtain a fair price for the desired services or products. The IT department is tasked with utilizing and incorporating third parties services and products to deliver IT services to the organization's internal and external customers.
The IT department ideally understands the strategy and demands of its customer community, together with the affordable budget provision. The Commercial department strives to achieve the best price, terms and conditions for the organization.
Service Levels Requirements required by the customer community need to be understood by the IT department and thus enabling services to be delivered to ideally match or exceed the requirements.
It may seem obvious, but...
...the Commercial and IT departments need to work together to understand the organization's requirements both today and the strategic future. In addition there is a need to understand the budget available and then identify the potential third party suppliers who may be able to deliver the services. It is important that the chosen third party supplier(s) must be able to demonstrate that they can work with the organization and possibly other third party suppliers currently employed.
I make the presumption that the organization has adopted ITIL best practices and that it expects all existing and future third party suppliers to also have adopted ITIL best practices.
Ensure that potential third party suppliers demonstrate and provide evidence and references from existing customers that they actively work to ITIL best practices and that the practices are mature. If a third party supplier does not demonstrate ITIL best practices to support the services they are offering, strongly consider walking away.