I've discovered an archive of shape models created from NASA and ESA spacecraft photography:http://sbn.psi.edu/pds/archive/shape.html
The models are in ICQ format which cannot be directly imported into Blender. However, I contacted Robert Gaskell and he provided a Fortran file to convert the ICQ file to Blender-readable STL format. I am attaching the Fortran file. I have not yet compiled the Fortran code. Perhaps someone can see how it works?
I also contacted Holger Sierks, the principal investigator of the OSIRIS camera on the Rosetta spacecraft. He provided a low-res OBJ model of comet 67/P Churyumov-Gerasimenko. I am attaching it, but it does not show very much detail. I hope to receive a higher-res model based on closer photography. I will post it once I receive it.
If you use the OBJ file for the comet in any animations or imagery for the public, please use the following credit:
Credit ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
OSIRIS was built by a consortium led by the Max-Planck-Institut für
Sonnensystemforschung, Göttingen, Germany, in collaboration with CISAS,
University of Padova, Italy, the Laboratoire d'Astrophysique de Marseille,
France, the Instituto de Astrofísica de Andalucia, CSIC, Granada, Spain, the
Scientific Support Office of the European Space Agency, Noordwijk, The
Netherlands, the Instituto Nacional de Técnica Aeroespacial, Madrid, Spain,
the Universidad Politéchnica de Madrid, Spain, the Department of Physics and
Astronomy of Uppsala University, Sweden, and the Institut für Datentechnik
und Kommunikationsnetze der Technischen Universität Braunschweig,
The support of the national funding agencies of Germany (DLR), France
(CNES), Italy (ASI), Spain (MEC), Sweden (SNSB), and the ESA Technical
Directorate is gratefully acknowledged.
We thank the Rosetta Science Ground Segment at ESAC, the Rosetta
Mission Operations Centre at ESOC and the Rosetta Project at ESTEC for their
outstanding work enabling the science return of the Rosetta Mission.