Picking templates- need reliable source

I need to show my design dept a large number of different themes to select from. I know how to make a child theme for the css file only. The other stuff like functions, header, etc are over my head. I’m looking for suggestions of the most reliable (stays upon security updates) template foundries/companies to but a theme from? This is my first real WP site. I’m a former Joomla guy. Jim