Several ideas come to mind with regard to your question. The U.S. Library of Congress is a gold mine of primary source information. As the telegraph lines made there way across the U.S. early telegraph offices were government operated. A good way to start is to locate the railroad stations west of Chicago and St. Louis. Early telegraph lines usually followed the rail lines ( because the land was already cleared). You can look through old city/town records by contacting the local historical societies for information regarding local railroad histories which most likely include telegraph histories as well.
In addition, the archives of newspaper publications can both offer you information as well as lead to to other sources.