The American Association of Radon Scientists and Technologiests (AARST) publishes a directory of radon service providers that are AARST members. The AARST directory is available here.
The National Radon Proficiency Program and the National Radon Safety Board are two national certifying agencies in the United States that list certified professionals.

Several states have regulations on radon service providers. Below is a list of states with licensing or certification requirements. Visit their web-site to see a current list of radon service providers.