You have to ask yourself one question: how accurate do you need this to be? are you making a tim burton skeleton-highly stylized, or a true anatomical rendition?
as someone in medical/scientific visualization, I can tell you that there are probably not going to be many free models of a full skeleton, and if there are they wont be very accurate.
Check the usual suspects-viewpoint et al...(and I just checked on 3dcafe, and the .dxf skeleton they have is no working right now.)
If you dont have to be too accurate, I'd say just load front and side images of the skeleton into your viewplanes, and start modeling with primitives. Refine as necessary, remembering that the proportions are more important than details because the skeleton is comprised of so many smaller parts that are usually combined in more stylized renditions.
