From Slavija bus to Belgrade airport, fly to Istanbul airport, then Metro to Istanbul. 4hrs 39min 106€. From Slavija bus to Belgrade airport, fly to Sabiha Gokcen airport, bus to Kadıköy, then ferry to Istanbul. 5hrs 42min 138€. From Belgrade bus to Niš then bus to Istanbul. 17hrs 52min 72€. From Belgrade bus to Priština then bus to Istanbul. 23hrs 52min 94€. From Belgrade drive to Istanbul. 11hrs 06min 180€.