Notes:
Digital maps come in many flavors. The most important distinction is between vector maps (maps represented by lists of points that are connected by lines in the mapping software) and digital raster graphcs (DRG) maps, which are digital images with known coordinates for each corner of the image.
The US Census has produced vector road and feature maps for the US, The USGS has made digital images of all the topographic quadrangles in the US. Topo quads as DRG files are available from multiple sources. In some cases they are made available for free by state agencies, in other cases they must be purchased from map resellers.