In addition to the traditional city transit systems which mostly started out as streetcar systems, there also existed a number of independent suburban bus lines, whose histories generally are considerably more obscure. And which were more challenging to research.
Many independent bus companies were sole proprietorships, or partnerships or family companies. The history is not always easily available for these companies, as privately held and thus not included in the Moody investment manuals. Usually a bus company could only operate a route, if it was granted a Certificate of Convenience and Necessity by the regulatory commission in that state. The state regulatory commissions generally issued reports, identifying each Certificate of Convenience and Necessity issued. Although the availability of such reports varies from state to state, and the availability of such information in the reports also varies. But many of the state regulatory commission reports included this interesting information. Each Certificate of Convenience and Necessity often was assigned a number, the numbering systems generally varied between states and were sometimes also used for railroads and utilities.
For many states however, such information is not available beyond the 1920's. One interesting resource for later information is the 1952 "Mass Transportation's Directory", offering an interesting "snapshot", of what obscure bus companies had existed then. The listings generally are detailedly descriptive of the nature of the operations, and communities served. Although some descriptions remain vague. Many of these lines came and went, and the listings in a "Mass Transportation's Directory" for a different year presumably will have a number of differences.
Another great resource listing obscure bus companies, is the Transit Badges web site. Transit badges are the main focus for that web site. But there are listings of many additional obscure bus companies, especially from the 1920's and other decades, whose existence might otherwise remain unknown.
The state regulatory commission reports and the Mass Transportation's Directory are referenced on various pages at this section. Mentioned are bus lines which have been identified as "suburban", along with bus lines generally operating in only one or just a few counties. Many of these bus lines operated beyond the regions which eventually approved creation of publicly owned transit agencies. As a result, service was usually eventually discontinued.
Bus lines operating over longer distances are considered as "intercity". And nearly all "intercity" bus companies were included in the "Russell's Official National Motor Coach Guide". This monthly publication was primarily for bus travel and ticket agents for Greyhound and other longer distance bus companies, and contained all intercity bus schedules throughout the United States and Canada.
This link is for the section covering past and present intercity bus lines, nearly all of which were listed in the Russell's Guide, and are not part of this section. Although in certain densely populated areas, the distinction is not always clear between suburban, interurban and intercity. The state regulatory commission reports include intercity bus lines, the reports from some states do not include services of a more local nature.