The US Dept. of Justice has an Antitrust Division[1], along with a page that details how and why[2] to get in touch with them.
The FTC has the Bureau of Competition[3].
Each individual state has an Attorney General office that will hear these types of complaints, and some of them have explicit antitrust divisions[4].
[1] https://www.justice.gov/atr
[2] https://www.justice.gov/atr/report-violations
[3] https://www.ftc.gov/about-ftc/bureaus-offices/bureau-competi...