From Slavija bus to Belgrade airport, fly to Istanbul airport, then Metro to Istanbul. 4hrs 49min 90 €. From Slavija bus to Belgrade airport, fly to Sabiha Gokcen airport, bus to Kadıköy, then ferry to Istanbul. 5hrs 52min 117 €. From Belgrade bus to Niš then bus to Istanbul. 17hrs 52min 60 €. From Belgrade bus to Skopje, bus to Otogar, then Metro to Aksaray. 19hrs 16min 76 €. From Belgrade drive to Istanbul. 11hrs 06min 174 €.