const
  HTMLID_FIND       = 1;
  HTMLID_VIEWSOURCE = 2;
  HTMLID_OPTIONS    = 3;

...

procedure TForm1.FindIE;
const
  CGID_WebBrowser: TGUID = '{ED016940-BD5B-11cf-BA4E-00C04FD70816}';
var
  CmdTarget : IOleCommandTarget;
  vaIn, vaOut: OleVariant;
  PtrGUID: PGUID;
begin
  New(PtrGUID);
  PtrGUID^ := CGID_WebBrowser;
  if WebBrowser1.Document <> nil then
    try
      WebBrowser1.Document.QueryInterface(IOleCommandTarget, CmdTarget);
      if CmdTarget <> nil then
        try
          CmdTarget.Exec( PtrGUID, HTMLID_FIND, 0, vaIn, vaOut);
        finally
          CmdTarget._Release;
        end;
    except
      // nothing
    end;
  Dispose(PtrGUID);
end;