You can change the icon for each Service post in admin -> Services, open single Post options and switch to Blog and single -> Single page. Please check the screenshot: 

Fontello source files are located in theme_name/css/fontello folder.  You can see the list of all icons opening theme_name/css/fontello/demo.html file in the browser. 

In order to add new fontello icons to the theme list, please use the following tutorial: