It's fairly easy to get Green stuff in Prague but I must say, that my experiences are not all good. Mostly it is because after the stuff cures it still remains flexible a therefore it is far from ideal for grinding (so making sharp edges is almost impossible). Also it always sticked more to the tools i sculpted with than to the model i was trying to convert (but that was most likely caused by my clumsiness )

I found out that much easier to use is a sculpey called FIMO. They use it to create beads or something I can't remember (but that doesn't really matter). It's not very sticky, so you can shape it with pretty much everything without worrying that it will stick too your tool and ruin the whole model. The only disadvantage of it is that in needs to be baked before it completely dries which may be harmful to blindbag ponies