Sometimes, like when building an accessory or hair, it is very helpful to have a reference sim body/head. These are obj files, they have no skeleton, no bone assignments. As such, they will not mess up your bone assignments or skeleton when you import them. Do NOT use them as a base for your body meshes, start with a fresh sims 2 mesh for that.
These are particularly useful for resizing hair meshes to fit other ages or sexes, because you can see where the hairline has to meet. You can import them, adjust your mesh, then be sure to delete them (in milkshape go to the groups tab and delete the entire group in one shot) before you save your work.
DJ Mur posted a set which is just heads (note that child heads and bodies are identical) and Dr Pixel posted a set with the body, head and scalp all as separate pieces which you can hide or show as needed. Remember to be sure to delete them (not just hide them!) before you save out your mesh!