This 1856 Map of Warren County, Ohio is what I am using to make the Township Maps. It is available free from the Library of Congress website in various formats, though none of the high-resolution formats will open in most common image programs. High-resolution image files are available from a number of commercial websites for $20-$50.
I have created a 15,530 X 15,205, 72 dpi, 43″ X 43″ jpg image file that can be viewed in any image software. This high-resolution image file is suitable for printing or whatever your needs. The file is 29.2 mb and is available for download HERE. (off-site link opens in new tab.)