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.
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