You can download them from Office Online.
http://www.pptfaq.com/FAQ00080.htm has links to them.
Once you've got them on your computer, one way to apply them is to go to the
Design tab and click the downward-pointing "More" arrow on the corner of the
Themes gallery, then choose Browse for Themes at the bottom of the expanded
gallery.
Navigate to your POT file (they show up, even though it says browse for
*themes*), select it and hit the Apply button.
You may have some minor issues with some of the slide layouts and need to
make minor adjustments, but they should work well for the most part. I do
suggest that you look closely at the color schemes and adjust those
accordingly and delete the extraneous color schemes also.
