I just turned up at Antalya otogar & bought the ticket at the relavent kiosk, the Bati Antalya & Fethiye Seyahat buses are roughly hourly & cost 18tl in March. You can pick up timetables for both at the otogar in Fethiye. These buses take around 3:30 - 4:00 hours to reach Fethiye & there's at least one comfort break up in the mountains.
The airport bus goes from one of the international terminals (it waits at the far end of the car park) & the domestic terminal (stop just outside the entrance) & takes about an hour to reach the otogar.