Classifieds Customization on Posts Page

Hi I was curious about where exactly the main classifieds listing page is generated from. I would like to add a sidebar and other details to the following page :

http://imageforthought.com/interlist/classifieds/

but am not sure what file to append to.

Also, when I click on an individual Ad, The ad Info does not show on the following page.
http://imageforthought.com/interlist/classified/number-2/

If I can find the php file with that calls the post values that builds each classified ad (or at least the index page) I can recreate those fields / display that info on other pages.

Ideally first I would like to start by customizing the main classified index page: http://imageforthought.com/interlist/classifieds/

Which file would I add to/ modify to do this ?