For many years, George’s Nafziger’s collection of orders of battle for conflicts dating from the 1600s to the 1900s have been a mainstay of the historical wargaming community.
Once available only commercially, Nafziger recently donated his collection to the Army’s Combined Arms Research Library.
The actual files are available as pdfs here. But as they’re numbered and not named, you’ll also want to consult the finding aid here. And a search function.