MyBB Community Forums

By default the 404 page looks dull and boring so I was wondering where I can find it in the templates to edit it.
Right now it's:

I want to change everything about it including getting rid of that useless search feature

Useless search form? That is quite possibly the single best element that can go in a 404 error page. I'm not going to go into details because it actually doesn't make a difference on a forum like yours, but on a typical niche forum I'd highly advise to keep it - it's good practise. Anyway, it's not in a template. It just throws an error message with a custom language string.

./inc/plugins/google_seo/404.php (line 214-228):

/* ---  Custom 404 error page --- */

/**
 * Create a custom 404 error page for errors that occur outside of MyBB.
 *
 */
function google_seo_404_page()
{
    global $mybb, $lang;

    if($mybb->input['google_seo_error'] == 404)
    {
        error($lang->googleseo_404_notfound);
    }
}

./inc/languages/english/googleseo.lang.php (lines 3-5):

$l['googleseo_404_notfound'] = "404 Not Found";
$l['googleseo_404_widget'] = "<script type=\"text/javascript\">\nvar GOOG_FIXURL_LANG = 'en';\nvar GOOG_FIXURL_SITE = '{1}';\n</script>\n<script type=\"text/javascript\" src=\"http://linkhelp.clients.google.com/tbproxy/lh/wm/fixurl.js\"></script>";
$l['googleseo_404_wol'] = "Seeing an <a href=\"{1}\">Error Page</a>";