Dunno if you’re talking about mine but it’s free and CC0 (don’t mind the free3D license)
You can download it here:
https://free3d.com/3d-model/accurate-human-skull-wip-35571.html
That said I would recommend @midoalageb to sculpt his own skull. Using a free model as the only object in an image is not going to teach you 3D modeling. Free models should mostly be used for scenes with many objects.
Looking at your skull, it seems you modeled it in edit mode. I really recommend to use the sculpt mode instead, as this kind of object is very organic and full of irregularities.