There is currently no established hub, that aggregates them, but public agencies often offer them on some open-data initiatives on a national level, e.g. Austria has https://www.data.gv.at/ and searching for "Gebäude" (building in German) lets you find e.g. the data set https://www.data.gv.at/katalog/dataset/ac74b38e-57cd-4c8c-8f... where you can download building footprints for the state of Tyrol.
The same thing is also done on an international level, e.g. the European Union provides platforms and also the environmental agencies in the US.
# edit:
Clarification: These footprints are not satellite-derived (that is the goal, but it doesn't work well enough for many applications, but we probably will get there ...), but are hand-crafted by people working in city planning. The point is you can use the data as training data.
