Istanbul's Emniyet Garajı bus terminal, at the southern end of Abdullah Çavuş Sokak southwest of Aksaray Square in Old Istanbul (map), serves buses taking passengers and freight to Balkan countries (Albania, Bulgaria, Kosovo, Macedonia, Romania), and also to the eastern Black Sea countries of Armenia, Azerbaijan, and Georgia.
The big operator here is Vardar Turizm, with buses going to Azerbaijan, Bulgaria, Germany, Georgia, Hungary, Iraq, Iran, Macedonia, Poland, Romania and Syria.
Other destinations and companies include:
Ast Turizm, Emniyet, Kesebirler
Nişikli Turizm, Vardar Turizm
Ast Turizm, Aybaki Tur, Golden Turizm, Mahmut Turizm, Mahmudoğlu Turizm, Metro Turizm, Nugo Turizm
Nişikli Turizm, Ortadoğu Turizm/Perla Trans, Özlem Turizm, Star Turizm, Troy Turizm, Vardar Turizm
See the page on buses to/from the Balkans for more information on some of these companies.
You may also want to read this personal report of a less-than satisfactory bus ride from Bucharest to Istanbul. More...