Istanbul Seyahat, at offices 67 and 68 of Istanbul‘s Büyük Otogar (main bus terminal), is one of Turkey’s major bus companies, with buses leaving Istanbul for towns in Thrace, Aegean and Mediterranean Turkey, including Afyon, Alanya, Ankara, Antalya, Aydın, Belek, Burdur, Çorlu, Finike, Gebze, Izmit (Kocaeli), Izmir, Kemer, Kumluca, Kütahya, Manavgat (for Side), Şarköy, Serik, Silivri and Tekirdağ.
They also have routes to Bulgaria.
Here’s their website. I tried making a reservation in its English-language section,but it didn’t work.
Telephone toll-free in Turkey: (242) 444 5959; or (212) 658 2626 at Istanbul‘s Büyük Otogar.
Alphabetical List of Destinations
Full List of Offices/Perons & Companies