There are few places to find a good consultant:

Before hiring an enterprise SEO consultant, it’s good to have a clear understanding of what you will need them for.

Remember they are one person with limited availability so be clear on:

At the enterprise level, payment is usually in a flat fee for a predetermined duration of time.

If a consultant is offering services only to be paid when an expected outcome is met (i.e., rankings for x terms, x links to site, x revenue increase), then they most likely aren’t familiar with the complexities and professionalism of an enterprise organization.