After you finish the first part of Umbacano's Quest, where you get the carving, and come out of the ruin, and it's deja vu where you are Indiana Jones and that guy is Belloq, and he asks you to hand over the treasure.... after the battle, the Khajit usually has an ungodly amount of arrows, the quality depending on your level.
I usually hit this quest after level 30 (for the added "I'm going to die at every turn" factor).
At level 30, on the Khajit, I find between 500 and 600 Daedric Arrows and a Daedric Bow.
I did this quest the other night at level 25, and this is what I found on the Khajit:
125 Daedric Arrows
150 Dwarven Arrows
111 Ebony Arrows
128 Elven Arrows
136 Glass Arrows
77 Iron Arrows
105 Silver Arrows
125 Steel Arrows
One Daedric Bow
FYI, for all those looking for arrows in Cyrodil, I have sources in the Shivering Isles as well. I just thought it was interesting to see the numbers and variety being different at levels.