Hi Martin,
sorry for the late reply, have been on a trip.
I bought the display from www.watterott.com, part no: LCD-09568
http://www.watterott.com/de/20x4-Zeichen-LED-gelb/gruen-serielle-Ansteuerung.
But they should be available also from www.tinkersoup.de or sparkfun.
Good luck with your project.
Axel