It coudl be in any one of up to 7 different files, depending on your theme, and how your site is set up. Of course, if your theme is set up in a different way then ‘standard” it could also be in a few other files as well as something stored in your database, so it’s going to be very hard to give you any idea of where to start looking.
As a starting point though, have a look at the template heirarchy to see how the template files work in with each other.
Even knowing the hierarchy, it’s hard to determine which template is used on any specific portion of a page. To help me out, I resorted to adding HTML comments containing the template name (outside of PHP blocks, after backing up the original template files) at the top of any potentially used template. (and bottom if necessary)
By examining the HTML source generated, it’s fairly easy to see which and where each template comes in at various places along the page by where my comments appear.
