Much depends on the type of dispute you're involved in. Many cities have community mediation centers that do an excellent job of handling most types of routine disputes (consumer problems, neighbor disputes, landlord-tenant fights). For more complicated disputes (business termination, personal injury, breach of contract) it is often better to turn to a private mediation center. Some organizations, like JAMS/ENDISPUTE, the American Arbitration Association, and Judicate, offer mediation services nationwide, while a number of regional groups do a good job. Private divorce mediations are usually handled by sole practitioners or small local mediation groups.
You can use a lawyer directory like Nolo's at lawyers.nolo.com, which provides extensive information about lawyer/mediators who advertise there. The profiles tell you about the lawyer/mediators's experience, education, and fees, and general philosophy. Or you can get a list from the phone book, but if all you have is a phone number, make sure to check the mediator's references carefully.