Is there a state agency that rewgulates dentists?

[Effete Snob]

2. What does it have to do with being a dentist?

The practice of dentistry is highly regulated. Your complaint does not seem to relate to the practice of dentistry.

The conduct of business is another matter. It sounds as if you have a general consumer complaint about accounting, which is likely more appropriate for the TN Consumer Affairs Division. Complaint form here:

https://www.tn.gov/attorneygeneral/working-for-tennessee/consumer/file-a-complaint.html

However, if you are having an ordinary billing dispute, then you will be told to file a civil claim with your county small claims court:

https://www.tncourts.gov/sites/default/files/docs/final_guidelines_and_helpful_info_for_people_in_gen_sess_court_-_may_2013.pdf