I've been working with TIGER/Line shapefiles of CDPs/towns/cities, and want to fill in the empty areas between these places with shapes of those unincorporated areas well. So far, I've been unable to find shapefiles of the unincorporated areas at a national scale. Are these available to download in one dataset?
Having worked with Census Bureau shapefiles for nearly seven years, I'm almost certain this isn't available by default. That said, the Census Bureau does provide Shapefiles of municipality boundaries.
You might be able to use something like QGis to load a shape file of all municipalities, then select the bounding area, and perform an inverse selection command using the differences geoprocessing tool, which would draw a polygon filling in anywhere inside the bounding box that doesn't already have spatial layers.