The Code of Federal Regulations contains the text of public regulations issued by the agencies of the Federal government. Proposed regulations and regulations issued so recently that they are not yet in the Code of Federal Regulations database, may be found in the Federal Register.
Rules and regulations pertaining to NHTSA are available by searching for 49 CFR 500. The Federal Motor Vehicle Safety Standards are available by searching for 49 CFR 571, and if a specific standard number of section number is known, then search for that specifically, e.g. 49 CFR 571.108 for the safety standard on motor vehicle lighting.