Among the highest quality DEM data you can get to go with a standard USGS 1:24K topo map is 10-meter SDTS DEM data, with heights specified to the nearest 0.1 meters (though the altitude accuracy of the DEM likely doesn’t warrant that resolution). I find it superior to the 10-meter NED data provided by the USGS Seamless Server, with fewer artifacts, although the SDTS format isn’t as well-supported as the DEM formats available at the Seamless Server. The SDTS DEMs are also in UTM NAD27, same as typical for USGS topo maps, versus the geographic projection for Seamless Server DEMs. There are a fair number of pay sites on the Web that offer these SDTS DEM files for a price, but there is one website where you can download them for free.
The GIS Data Depot has gone through some changes over the years, but is still around. It offers not just SDTS DEMs but a host of other GIS data, some free, other not free (you can often find the paid data for free elsewhere on the web). Data is classified as “statewide” or “countywide”, and you’ll find the SDTS DEMs in the “countywide” sections, listed in the Digital Elevation Models (DEM) – 24K section under the 1:24K topo map name whose area they cover (offered in both 10-meter and 30-meter resolutions) . The catch with the free “Normal” downloads is that they are at slower speeds than the paid “Premium” download (appears to be roughly 10 kilobytes per second or so).