Here are some good places to look for already created presentations for you to use. There are also resources you might use in making your own.
Pete's Powerpoint Station great source for FREE Powerpoints by other teachers in all subjects
http://www.animationlibrary.com/ Free animation for Powerpoints
Flickr: Creative Commons images with fewer copyright issues
Google Images - hunt for pictures for powerpoints easier
Jefferson County Schools - Great school district site with sources you can use k-12 all subjects