Wondering how I picked the sources I did? I know these template sources best. They are companies that have been around for quite a while, have good customer service and reasonable prices for their templates. In fact, a few of the places listed even give away their templates free!
http://www.powerpointbackgrounds.com
http://www.soniacoleman.com
http://www.powerpointed.com
http://www.brainybetty.com
http://www.powerfinish.com
http://www.powerbacks.com
http://www.templatecentral.com
http://www.presentationpro.com
http://www.powerpointart.com
http://www.power2present.com/mainframe.htm
http://www.inzones.com/powerpoint/index.htm
If you need even more template sources, go to Google (or your favorite search engine) and do a search on "PowerPoint Templates." You will get more results than you'll have time to investigate!