What you are looking for are DEMs. Digital elevation models. Most cities have ones created be synthetic aperture radar, accuracy about 1 meter, or lidar, accuracy about 1 foot. Rural areas are a lot more variable. The shuttle flew a mission that did SAR from orbit, but the grid spacing is about 30 meters, and the accuracy is awful.
If USGS topo data is good enough, I vaguely recall a page on converting topo lines to terrain. Warning: topo data is not very good at the house lot size level. There is typically a semi-systematic error in the horizontal placement of contour lines equal to several times the contour interval. The shape is right, but slope inflections tend to be moved away from the centre of the photographic track.
If its critical to get it right, rent a DGPS, and take a waypoint every 10 meters. If the terrain isn't too bad, you can do a hectare an hour.