Sketchfab is a large platform with many user-uploaded 3D models, including those in the medical field.
TurboSquid has a category for medical models, which may have resources relevant to NIH models.
Embodi3D offers access to both 3D models and medical imaging resources.
Click here to browse free 3D models!
Tinkercad is a great tool for modeling with polygons and existing .stl files.
For information on the conversion of medical imaging into 3D models, try reviewing our libguide!