It's easy to buy tickets at the station when you get to Turkey. Most major stations have a computerised ticketing & reservation system, so can book any journey in Turkey. It's not usually difficult to get seats or berths on the day of travel or a day or two before, although sleepers can sometimes get fully-booked. Alternatively, you can buy tickets in advance either online or via a TCDD-authorised agency as shown below. Stations accept Turkish lira cash, and may now accept credit cards. Booking for Turkish domestic trains opens 30 days before departure for some trains, 15 days ahead for others. The high-speed trains open 10 days before travel.