I am looking to find a dataset/shapefile of a Building footprint/outlines for the city of Detroit. I have found this dateset of detroit parcels but I need Building Footprints!
A fairly recent map has been made showing the buildings footprint for Detroit as of April 2010:
The dataset used was from SEMCOG (Southeast Michigan Council of Governments). As @AdamKara mentioned, you could ask if you could gain access to the dataset.
Note that they may charge you if you want to use it or indeed refuse completely.
When looking for datasets for any municipality, you should search for the keywords "GIS data download" and include the city's name. However, must cities do not have dedicated GIS sites up so an even better bet is to run that search with the county's name. Most counties with a major city have a GIS site up with their features available for data download. It does not appear Wayne County Michigan has a site up which means if they do have the data they will likely have an FTP site with the information they have available. I would contact the county's GIS coordinator and ask if they have the file. I manage GIS systems for over 20 communities and if a search doesn't find it the best option is to contact them directly and ask!
SEMCOG doesn't charge, they have an open data portal e.g. http://maps.semcog.opendata.arcgis.com/datasets?q=building&sort_by=relevance