There are several good general sources for 3D-model, just google "free 3D model" Pick .OBJ and .3DS with a reasonable polycount.
If you want models that are low poly enough for Skyrim but need to be converted first. Have a look at TES Nexus TesAlliance etc. for modder resources that allow you to use them in other games (or ask the authors for permission). http://www.morrowind-mod.com/ lists all the treasures of old, some links are still working, get them while they're hot.
If you ask for Skyrim-ready files you will only find them as resources on the usual places (TES Alliance, Skyrim Nexus etc.) but there aren't many right now.
Creating an animated bat as a creature is impossible right now, as there are no exporters for any 3D-modelling program, and animation info won't carry over (AFAIK it can be done but only with intimate knowledge of both, the modelling program's file format and the NIF/KF format.
Edit:
Yet another old site has gone. Morrowind-mods.com has expired

I visited there just a few weeks ago, and now it's gone. It's been my first stop to look for models for a long time.
Yet the web never forgets (keep that in mind: generation facebook):
http://web.archive.org/web/20110623022705/http://morrowind-mod.com/