SAT NAV SCREEN LEXUS RX300?

Hi. I have a Lexus RX300 2005 and the touch screen on the sat nav / muliti media system has completely stopped working. It went a bit funny for a while e.g. when you press a button for one thing it would do something else but now the buttons on the touch screen have froze completely. I have spoke to Lexus they say I need a new unit ouch. Any ideas of how to fix or get a 2nd hand one. Thanks