Try the built-in Unity asset store. When inside Unity, open window -> asset store. Or ctrl+9. I think there are some free models there.
Otherwise, you must use 3D max or Maya or similar programs for 3D models to export to Unity. There are lots of free 3D models online for those. So download the free models then export them to Unity.
Hope that helps..
answered Feb 10 '11 at 09:37 AM