can't find includes content

I have a template page with an include that looks like this:

<div id="rightBottom">
     <?php get_sidebar(); ?>
</div>

However, I can’t find the file that supplies the content for the included on the rendered page.

Where or how can I find where this is coming from? Is there a parameter on the parent page that defines this or something?

I opened every file in my theme and did not see the content.

Thanks,
Justin