Charger with long usb cable
If you cannot find a reference to one on the Asus website, then Asus does not make one. Also, longer cables cause loss of current and hence your phone will charge slower.

Most companies these days don't even sell their chargers and cables in open market. They are only bundled with the products and available via authorized service points. So, if you have an Asus point nearby, go check in there.