Do I need to use an "NRTL" for my final NEBS testing?
Not necessarily, however, it is strongly recommended. The RBOCs like OSHA's program that approves these labs. OSHA oversight provides a governmental body's third partly approval of independent test labs. Keep in mind that OSHA approval as an NRTL only applies to safety testing (personnel and fire). The thinking is that since the NRTLs are covered by a strict governmental program, testing to other than safety is as well regulated. Maybe and maybe not, that's just the thinking.
To get more information on who's on OSHA's list, go to this site:
OSHA's List of NRTLs
Note that not all of the labs listed on this page perform NEBS testing.
Some RBOCs will allow other non-NRTL independent lab testing. It's best to check with each RBOC to determine what is acceptable.