What you really want is to know where you can specify a custom 404.html landing page.
To do that you need to edit the <error-page> section of 'Web.xml' with text like the following:
This file exists in several locations such as CA\SC\Tomcat\Webapps\Manager, CA\SC\Tomcat\Webapps\ROOT and CA\DSM\WebConsole. as well as copies in other locations for CIC, ENC, Patch Manager etc. You just need to update the proper one for your needs.
Once updated and saved, you can recycle Tomcat via 'CAF TOP TOMCAT' followed by 'CAF START TOMCAT' run from an administrative command prompt and then you can test your changes in a web browser.