As far as I know, there's no such thing outside of weapon bits, the only thing that exists are complete proxies that you could perhaps cut to keep only the head, like here. Otherwise you'd have to commission an artist to make you a head stl tohaa. There's also the solution of making a mold of a tohaa head with blue stuff greenstuffworld and replicate it, but the definition won't be as good so you'll probably have to resculpt the result.
