A digital elevation model (DEM) is needed in BRAT to help us estimate stream power and a hillshade derived from the DEM will give us cartographic context. In principle, you could use any DEM that covers your entire watershed of interest. In practice, a 10 m DEM will be more then adequate for estimating reach slopes and for estimating contributing drainage area for each reach. The USGS's National Elevation Dataset
is broadly available and will work for our purposes. Of the many places you can download a DEM for your study site from, we find the USDA's Geospatial Data Gateway
the simplest, quickest and most reliable.
The video below walks you through the download, mosaicing the tiles together and deriving a hillshade.