To locate an evaluator, check out these national directories:
NEW! ATE Evaluator Directory (a work in progress as evaluators add themselves to the directory)
American Evaluation Association's Evaluator Directory

To post an evaluation job or request for proposals:
American Evaluation Association Career Center
 

For guidance on negotiating agreements and contracts with evaluators:
Evaluation Contracts Checklist (Daniel Stufflebeam)
Checklist for Negotiating Agreements to Evaluate an Educational Programme (Robert Stake)
General Tips for Hiring an Evaluator (Kellogg Foundation)

For information about what evaluators should know and be able to do:
Essential Competencies for Program Evaluators (American Evaluation Association)
Evaluator Competencies (International Board for Standards for Training, Performance and Instruction)
Competencies for Canadian Evaluation Practice (Canadian Evaluation Society)
Self-Assessment of Program Evaluation Expertise (Daniel Stufflebeam)