There are many places to find themes; the list that follows is not exhaustive. A Google search for "Magento theme" will reveal more suppliers. The following list is ordered by number of themes available; the first in the list has the most themes.
Templatemonster (http://www.templatemonster.com/magento-themes.php): Here you will find a wide range of wildly differing themes. Unfortunately the quality of the themes can differ widely too, so make sure to at least use the live preview function.
Themeforest (http://themeforest.net/category/ecommerce/magento): This site has a good system in place that allows you to recognize the good themes (based on number of downloads and ratings), and themes are priced relatively cheaply.
AheadWorks (https://ecommerce.aheadworks.com/magento-themes-and-templates.html): This site is a quality extension builder that also sells some decent Magento themes.
Polarthemes (http://www.polarthemes.com): Here, only a couple of themes are available...
