I spent hours on my first play through, looking for 30 of them. What a pain in the tail. I ended up going into blackreach, picking the ones I find along the main road, leaving, and then hitting the same spots again later in the game. On PS3 I was able to put them down or into storage, while I waited to find all 30. If you get a glitch, where you put 5 in storage and it sends your count into negatives, pick them up and put them away one-by-one, instead of all at once. It seemed to fix that for me.