In order to find themes and templates, you first need to decide how you are going to build your website. Are you going to use HTML or a CMS such as WordPress and are you looking for any specific features?
Your best friends here are search engines such as Google and Bing. If you are looking for a WordPress theme in order to build a classified ads website, you can type this into a search engine and can view thousands of pages if necessary.
Themeforest.net is also a great resource to find themes and templates, however, these are generally premium themes.
A CMS such as WordPress also has its own database of free and freemium themes to search and this can be done directly from within our WordPress admin area.