Theme Mart has both free samples and low cost themes available for download. EB ThemeShop offers a very low cost membership where you can download themes as you need them. There's also a few FREE downloads. Captivations Theme Mall has plenty of themes. The available themes were created by many different designers.
Some themes are free.
FrontPage Stars WebRing has few free themes available for download. They also maintain links to other resources. PixelMill offers several free themes as well as themes for a small fee. They also have templates available. Paul Vineburg's site has free themes and themes for a small fee.