Trucking companies are legally called "motor carriers." Motor carriers that are registered with the Federal Motor Carriers Safety Administration (FMCSA) are issued "operating authority" (commonly just referred to as "authority"). The FMCSA has a publicly searchable database containing information about motor carriers including address, phone number, registered agent, and more.
The main FMCSA search web site is called the Safety and Fitness Electronic Records System, or SAFER System. The address is https://safer.fmcsa.dot.gov
The various searches that can be performed are in the middle of the page under "FMCSA Searches". For your convenience the most relevant ones you might want to use are listed below.