Though not a “marketplace”, such as ThemeForest, with multiple theme authors, Elegant Themes is well established and worth checking out.
With access to over 75 themes for $39/yr, a growing feature list and eye for design, they are worth checking out.
For the all-inclusive price of $39, you are only paying $.50 per theme!
You are also free to use their themes to build websites for your clients.
Lots to learn as you get started – $39 is hard to beat. Check out their Gallery and see if anything strikes your fancy.
It’s hard to limit what can be said about Orman Clark and the team behind Themezilla. Why? Because he is also the guy behind PremiumPixels.com (free stuff for aspiring designers) and overall good design.
I’ve personally used or worked with a number of Orman’s themes (now “Themezilla’s Themes”) and can vouch for their ease of use and customer support.
It’s a good place to start if you want to start somewhere other than ThemeForest.
They also have some plugins worth checking out.
ThemeForest, and the entire Envato Marketplace family, has changed the way many build, access, and approach web construction, design, and extension. It’s a win-win, for users and designers, developers and owners.
In their words, you can begin your search for:
Site templates and themes to skin popular CMS products like WordPress, Drupal and Joomla, starting at $5.
It’s a great place to start for WordPress Themes, and I recommend it.