The official WordPress theme directory is where most people find a design when they first start using WordPress. It's a fantastic place to find a WordPress theme, but when you want to build something more professional, you will generally find better solutions in the premium market.
Premium WordPress themes tend to be sold in one of two locations. They either sold directly via the developer's own shop or they are sold via a WordPress theme marketplace.
Marketplaces sell WordPress themes from thousands of different designers. In exchange for generating a lot of traffic, they take a percentage of any sales that are made.
From a user's point of view, marketplaces are a good place to search for themes as there are thousands of designs to choose from. Since these designs are created by so many different authors, you get a really good variety of designs.
In this page you will find a list of marketplaces where you can purchase WordPress themes.