As the name implies, a computer bag is perfect for carrying your laptop. You can carry it in several different ways, including a detachable shoulder strap.
From Aimact
Case Logic VNAI-215 Black 39.6 cm (15.6") Sleeve case
Technoworld PLC
Case Logic VNAI-215 Black notebook case 39.6 cm (15.6") Sleeve case 3201492
OnBuy
Case Logic VNAI-215 Black notebook case 39.6 cm (15.6") Sleeve case