From Slavija bus to Belgrade airport, fly to Istanbul airport, then Metro to Istanbul. 4hrs 54min 94 €. From Slavija bus to Belgrade airport, fly to Sabiha Gokcen airport, bus to Kadıköy, then ferry to Istanbul. 5hrs 47min 90 €. From Belgrade bus to Niš then bus to Istanbul. 17hrs 52min 62 €. From Belgrade bus to Skopje, bus to Otogar, then Metro to Aksaray. 19hrs 16min 77 €. From Belgrade drive to Istanbul. 11hrs 45min 167 €.