There are plenty of interesting templates on the CD that accompanies this book, and many more are available online. Here are some sources to get you started:
PowerPoint Backgrounds: www.powerpointbackgrounds.com/index
Sonia Colema: www.soniacoleman.com
PowerPointed.com: www.powerpointed.com/001100/back/
PowerFinish: www.powerfinish.com/indexl
You can always search regular search engines online for "PowerPoint templates" or "PowerPoint backgrounds."