Most of my exotics have come from Crota. I think only one from from nightfall which I do with three characters every week. Try doing more of Crota maybe? You get a number of points in each raid where there seems to be a good possibility of exotic drop. Someone in the team always seems to get something reasonable to excellent. My Gally came from Crota a few weeks back and a guy last night got one. I've also picked up MIDA P&T Truth NLB(!) Plan C (many times) and some others from the raid.