The comment.tpl.php is located @<drupal-root>/modules/comment/comment.tpl.php. If you want to customize it for different content type. Copy it to your theme folder and rename it to

• comment–node-<content-type>.tpl.php

For example, the machine name of Article is article. So the file name should be comment–node-article.tpl.php.

Done =)

Reference:

• StackOverflow – Drupal 7 comment.tpl.php overwrite for specific node is not working
• Drupal API – comment.tpl.php