Just got this fairly cheap but ok wall-to-wall carpet from Hornbach. It measures 5×5,5 meters so quite tricky to get in the room.
The carpet is a short pile velour carpet and the whole piece (about 25 square meters) costed only about 400 euros wich must be quite cheap I guess.
They had some other colors too (see below) but this dark blue/dark violet seems best for the room I am making. I will paint the walls dark violet too.