I was just messing around in srm.conf and ran accross the following lines:

ErrorDocument 401 /cobalt_error/authorizationRequired.html
ErrorDocument 403 /cobalt_error/forbidden.html
ErrorDocument 404 /cobalt_error/fileNotFound.html
ErrorDocument 500 /cobalt_error/internalServerError.html

Is this what I want to change to make new default error pages across the entire RaQ? And if so, where is the cobalt_error directory?

Yes, that is what you want to change. :)

I looked around on a Cobalt here and I found the /cobalt_error/ directory to be alised to "usr/admserv/html/.cobalt/error/". In there, I found all of those files. :)

It might be easier to put the "ErrorDocument 404" line in an .htaccess file, and put the .htaccess file in the /home/sites directory. That should cover the whole server (unless a virtual host has their own .htaccess file).