PowerPoint-search.com searches PowerPoint files as per keyword enetered. You can also see last 10 searches.
Slide Finder not only searches PowerPoint files as per keyword enetered but also provide you information about file size, thumbnail preview of all slides and presentations similar to your search keyword.
SlideWorld let you rate any PowerPoint presentation and displays total number of downloads for that particular presentation.
You can give a try to all of these PowerPoint search engines and choose the one which finds best PowerPoint according to keywords used in search.
Some other resources from where you can download PowerPoint files