Where can I find a comprehensive list of all health insurance companies/providers in the United States?
Because health insurance is regulated by each state, there is no single national list. Health reform (Affordable Care Act/ Obamacare) added lots of federal regulations to health insurance, but the actual oversight of insurers remained with the states. You could compile one by visiting each state’s insurance regulator website and downloading the list of licensed health insurers. The National Association of Insurance Commissioners website has a list of each state’s regulator, but not a link to the state’s website.
If you did compile a list of licensed insurers from each state, your list would not include employers who self insure their health plan; these employers are providers of insurance though they only cover their own employees and their families.