1. Themeforest.net – Theme Forest has over 2500 WordPress themes so their selection is second to none. The downside of using ThemeForest is that some themes are better than others. There are plenty of themes that are poorly supported, out of date, and do not provide good documentation for setting up the theme.
2. Elegant Themes – This company only carries 81 themes and haven’t added to this number significantly. As they add new themes I think they discontinue old themes. The great part about this site is that all themes are well documented, supported, and generally high quality. If you are an agency you can access all of their WordPress themes for the cost of one. Also the consitency between themes can make things easier after you are familiar with one of the themes.
3. Template Monster – This company has over 1500 WordPress templates that are all professionally developed and documented. The downside of this site is that their themes are expensive. Also, their themes are created by a variety of developers so just because you know one template monster theme doesn’t mean you will know how to use another.
4. WordPress theme directory – When I first used WordPress I almost always use the free themes available here. It wasn’t until I bought my first theme that I realized that I could save a lot of time by paying for a $50 theme and improve the quality of the websites. There are a few good themes in this repository but in general they are low quality, and because they are free their support is limited.