They should be /includes/templates/your_template/common/tpl_header.php and tpl_footer.php, where your_template is the foldername of the active custom template.
As this is apparently a highly modified installation, it could be something else entirely; we would have to see the file structure to be sure.

Use FTP to access the Zen Cart installation, download the files, edit and re-upload them. Note: you cannot use Word to edit these files, as it will add extra formatting and junk that will damage the files. Use a good text/file editor like Notepad++.