I believe the only direct buses are overnight long distance services - someone may know better.
However there are plenty of buses between Marmaris and Fehtiye and then between Fehtiye and Kalkan. The changeover point is Fehtiye Otogar (bus station) which is relatively close to the town centre if you fancy a short diversion. There is often a complimentary bus service running in conjunction with the bus from Marmaris.
The last bus from Fehtiye to Kalkan is at 18.30.
One word of warning, don't be tempted to use dolmus's for any part of the journey as they are very cramped and take ages. Use the small or large coaches.
The Otogar in Marmaris will be able to help you with exact times and fares plus make reservations for you although generally speaking advanced reservations are not necessary and have the disadvantage of tying you into a particular operator and specific bus times.
Hope this helps.
Alan