Does anyone know where to download a shapefile containing the locations of US interstate and highway exits?
I might suggest using OpenStreetMap and the highway = motor_junction tag/value pair.
From Overpass Turbo, use the wizard to build and run a query using that combination, and you'll find you get a decent result set:
You can either export that result to GeoJSON (or other formats), or use the query in QGIS to download the data directly to your desktop.
One source might be the census: https://www.census.gov/cgi-bin/geo/shapefiles/index.php?year=2017&layergroup=Roads