From Offices 71 and 72 of Istanbul's Büyük Otogar (main bus terminal), Süzer buses go to the Black Sea towns of Afsin, Arakli, Giresun, Of, Rize, Samsun, Trabzon and Yomra.
Artvin Turizm buses go from Istanbul to the eastern Black Sea coast and Kaçkar Mountains towns of Ardanuç, Borçka, Hopa, Rize, Savsat, Yusufeli
Resadiye Termal buses go to Amasya and Turhal.
You can easily find tickets to all available destinations here.
Alphabetical List of Destinations