From Slavija bus to Belgrade airport, fly to Istanbul airport, then Metro to Istanbul. 4hrs 44min 90 €. 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 77 €. From Beograd train to Sofia then bus to Istanbul. 43hrs 25min 62 €. From Belgrade drive to Istanbul. 11hrs 06min 172 €.