For third-party modules for Windows, my go-to resource is Christoph Gohlke's Python Extension Packages for Windows. You can find the latest version of Pillow here. Make sure you're working with the python.org version of Python.
As far as your specific error, it's hard to tell exactly without a traceback, but make sure your script is calling the correct version of Python. If you have also installed Python 2.7, for example, your script may be calling that instead of 3.3.