The problem with most free topo maps is the contour interval... Most of them are like 10-foot or even 25-foot contours. That's fine if you're in southern Ohio or somewhere else with really steep, pronounced terrain. But in the flatlands that information is basically worthless. The best option is some GIS software and free lidar data, with which you can create your own custom contour interval. But that requires knowledge of GIS software and processing, which most folks don't have. Thankfully a lot of county auditor websites now offer some damn good online mapping interfaces.