Authoritative contour data (and many other datasets of interest) can be downloaded from the US Geological Survey's National Map page. Select the 3D Elevation Program to access a view like that shown below. For contours, check the box in the left pane and then use the mapping pane to search on a particular Address/Place ("st louis, mo" is shown in the example) or you may use one of the other options by selecting above the map.
Once your map pane is showing the area of interest, click on "Find Products" (either the link in the pop-up or the button over the left pane) to return a list of available datasets that will look like this...
Use "Footprint" to see the spatial extent of this dataset and click Download to initiate transfer to your local system. The file you receive will depend upon the listed Format of the resource, in this case it is a Shapefile. ArcGIS will open Shapefiles and you can directly add the contour lines to your map document.
You can also download DEM products using this interface...instructions for processing DEM files into contour lines will be posted soon.
Last update: 9/2/16 Bill Winston