It's 5mm thick extruded Polystyrene (expanded would be Styrofoam) laminated with paper on both sides. Usually you can get something like this in art stores and even some architects use it to build models.
There is one type of foamboard called "Kapa" which is made from Polyurethan and about twice as heavy as the Polystyrene stuff.
I am also living in the Czech Republic (Englishman in CZ) and I was looking to build a plane from foam board. Did you manage to find any places that sell it here or did you have any luck with the stuff on ebay? I found a place that sells it in Prague but its the Kapa stuff, I am not sure if its too heavy http://www.promodely.cz/products/kapa-bila-1000x700mm-5mm/ The official name seems to be Kapa Line white and it has some chromokarton surface for indoor use, meaning its probably some kind of paper.
I would be interested to know if you managed to sort anything out, I suppose if the stuff on ebay is decent then it might be worth buying.
I don't know how Amazon works out for you guys in CZ. Here is some foam that is the same as the DTFB used by FT. It has different colors and the prices are always fluctuating. But, if shipping and cost works out for you guys, then this might be worth a try.
I ignore the color (it is only on one side) and order the cheapest color available at the time of order. I have used the black and the red. I have used sheets of this and sheets of DTFB in the same build (a buddy mailed a case of DTFB to me), and they are the same weight, and size.
Thanks for the info TEAJR66, not sure if getting it from Amazon would be so straight forward. I would rather like to source it from somewhere in the Czech Republic if I can to avoid the large shipping costs. Its a real pain I wish we could get DTFB here. My other idea was to get the 3mm Kapa line which might work out about the right weight, but maybe its too thin.