There are 168 bus company offices and departure gates/platforms (peron) at the Istanbul International Bus Terminal (Büyük Otogar).
Many of the companies also pick up passengers at the Harem Otogar on the Asian shore of the Bosphorus.
Here's how to buy bus tickets in Turkey.
See my list by destination (ie, Istanbul to Ankara, Istanbul to Antalya, Istanbul to Bodrum, etc); my page on Turkey's best bus companies; how to go from Istanbul to Cappadocia; and the pages on buses to Greece, Bulgaria and the Balkans, and Europe.
Büyük Otogar (Istanbul International Bus Terminal)
Buses from Istanbul to other Cities
How to Buy Bus Tickets in Turkey
How to Go from Istanbul to Cappadocia
Buses to Bulgaria and the Balkans