The Registry of Interpreters for the Deaf (RID) is the leading organization to nationally qualify sign language interpreters in the United States. Some other certifications are allowed in lieu of RID certification, depending on state laws and the hiring organizations.
If do an agency search within RID's database, remember the list is limited to organizational members. Many certified interpreters contract with agencies that aren't organizational members so not seeing an agency listed doesn't necessarily mean they aren't credible.