In searching online for reports from the various state regulatory commissions, or in general when searching for books which can be downloaded, often the year 1923 seems to be a benchmark year. Which perhaps relates to when certain materials typically become public domain. The reports from the state regulatory commissions usually are more readily available for before 1923. Meaning that less information is typically available for the states which adopted regulation later than the other states.
Where information was not available from state regulatory commission reports, early bus history for some states is from the publication "Bus Transportation", issues from 1922 and 1923. Though that information does not include the names of the bus operators. These were mostly smaller bus companies, and usually it was not possible to establish how these companies became part of the Greyhound or other larger bus companies.