There are buses from:
Kavarna to Kamen Bryag every day at 7.05 and 15.45 (3.45pm).
Durankulak (on the romanian border) to Kamen Bryag there aren't any buses.
Shabla to Kamen Bryag there are buses only to Tyulenovo which is less than 5km away. They are Mondays and Thursdays at 6.50 and 13.30 (1.30pm)
Kavarna bus station number is 0570/ 82390..although it might be useful only if u speak bulgarian
The best is to have a car.
Some friends come on motorbikes some hitch-hike (it's tough though, because there aren't many vehicles passing by). Or it used to be like this. I find out that more and more people go there during the summer ( some buld summer houses unfortunately).