that my friend is plasticard as they so call it! you can find it at your loccal "hobby" store, ie. model trains, airplanes, radio controled cars etc
now your icon shows you are in Netherlands so I am not sure where you would find it, there are several companies out there that provide them.
It really is hard to point to a good webiste because I have never brought them online, it really is best to walk into a hobby store and pick out what it looks like once you seeit.
http://www.evergreenscalemodels.com/
is a website that supplies my local locomotive store
good luck finding what you need! and WAAGH! on! I am going to attempt to turn a scale model tank into looted wagon soon !!!