Some potential criteria to consider:
- Local (to avoid travel expenses)
- Willing to use e-audits/virtual audits? (see:
http://www.amazonconsultinginc.com/auditing/)
- Do they need industry experience?
- Are you focused primarily on compliance to ISO or will you also want broader evaluations?
- Any required education or auditor certification?
I'd definitely start by contacting your local ASQ section.