Alstom (without the 'r') is a big European train manufacturer. I'm not aware of them building buses as well, but I guess a bus seat is pretty much the same as a train seat so it wouldn't be surprising if they supplied seats to bus manufacturers.
Even if you could get them, I suspect a replacement set of OEM headrests would cost far more than taking your existing ones to your local upholsterer to be fixed.
Jeremy