We booked a ticket from a company named Oncu for 120TL one way, 240TL total from Otogar, Istanbul. You can reach Otogar by Metro.
The bus was comfy but it's a 10 to 11 hours ride so u need to get a neck holder or a pillow so you can get some sleep.
They stop 3 times for rest so there was no wasting time, the drivers were good too.