Car For Sale Signs

Any papeterie will have it, or you can print your own from autoscout if you also put the car for sale there:
hhttp://ww2.autoscout24.be/service/service_manuals/as24_service_manuals_l...
As the page warns, you are legally not allowed to put the sign on when the car is on the public street (neither moving nor parked). This is the law, the practice of course is a different thing :)