I discover today that option to disable touchpad during writing in KDE 4.8 is missing. Where I can find this option?

KDE upstream has to my knowledge never shipped a module allowing you to configure the behaviour of the touchpad. However, several 3rd party modules do exist on kde-apps.org, and some of them have been shipped by distributions. I suspect it is this you are referring to.

You probably want to search for "synaptiks" or "ktouchpad" in your package manager, and install the relevant packages if they are found. You may need to check with your distribution to see what the actual name is of any package which handles this in KDE if they ship it.