The Sketchfab collection seems to have been set to non-downloadable. The morphsource collection can be downloaded, but only as an image sequence, not as 3D meshes. Fairly easy to convert images sequence to mesh, but I wish they had made the task more straightforward.
There are meshes in morphosource too. Didn't try downloading any but they are available as STL files with 1-2 million polygons per model. Some meshes are individual bones or skulls, others are complete specimens. The interface is a bit clumsy but you can use "mesh" in your search string