Several bodies of knowledge include information about how to select vendors for providing services:

• PMBOK v3 chapter 12, "Project Procurement Management"
• COBIT AI3.1 "Technological Infrastructure Acquisition Plan," AI5.1 "Procurement Control," and DS2 "Manage Third-party Services"
• ITIL v3 Supplier Management

However, the most thorough body of knowledge we've found so far is Carnegie Mellon's "IT Services Qualification Center." They have created "eSCM-CL," a capability model for client organizations interested in "eSourcing."