Here is a list of sites that offer premium themes for WordPress.
Elegant Themes
Elegant Themes offers very professional and elegant looking themes. For only 19.95 a year, you can full access to current and future themes. I have a subscription here and think it is well worth it.
Theme Spinner
Theme Spinner offers a wide range of attractive widget-ready, customizable and AdSense integrated WordPress themes at affordable prices.
WooThemes
WooThemes offers cutting edge WordPress templates with clean layouts, click color pallettes, and neat typography.
iThemes
iThemes produces high quality premium WordPress themes for photographers, designers, and businesses.
