Getting the correct theme can be quite a difficult and sometimes overwhelming task. You may find that you are committing long-term to a theme so its always good to shop around, do your research and find one that best fits your requirements. Understandably and especially for beginners, choosing the right theme is a complex operation. Having worked as a web developer I can fully recommend the following places to purchase themes.

Envato Market - Themeforest

Themeforest is the most popular destination for people buying a WordPress theme and has somewhere over 8000+ themes to choose from.

Template Monster

Template Monster has much less themes available than Themeforest but nonetheless there is a good selection to choose from. With over 1200 to browse through, you will surely find something that you fancy

StudioPress

Studiopress has excellent themes available built using the dependable genesis framework and this very site is built on a StudioPress theme. You can choose from a selection of generic child themes and these are all fully customisable. In fact, many web agencies recommend and use StudioPress themes in their business.

GeneratePress

GeneratePress is a more simple theme developed by Tom Usborne. However, don't let its simple look fool you, available in a free and paid version,  its a simple, lightweight & responsive WordPress theme available for free on WordPress.org and is extremely popular. With the addon package available at a very reasonable price, this is a theme that you can have plenty of fun practising with and again is a very popular choice for web developers as its highly customizable and works well with page builders.