Haha, I know exactly how that goes!

The fast way to find out which specific file to edit is by using the Developers Toolkit in your Admin:
http://www.zen-cart.com/content.php?...-files-to-edit
http://www.zen-cart.com/content.php?...lopers-toolkit

You'll find that the text itself all comes from language files, whether for the individual page/module that's building that email, or from the email_extras file which contains some common email texts.

The HTML email template/layout/design files (not the text content itself though) are in the /email/ folder.