Try /includes/templates/your_template/index/tpl_main_page.php .

Unfortunately, this will apply to all index pages including category listing and product listing.
For editing home page layout, you will need to add/modify code in the main tpl_main_page.php, with tests for
PHP Code:
if ($this_is_home_page){
  //do home page stuff
}else{
  //do default stuff
} 
The CSS can be customized for the home page by adding a file named index_home.css; but adding an /index_home/ folder will have no effect.