Greys never dropped them for me... didn't get the game at release so I don't know what may have changed.
Trees are pretty rare drop but they still do.
My favorite is to do crypts in the Black Forest.. nice getting a half stack from one.
A lot of people wait for rain days and shoot birds then... birds will stay grounded when it rains. Also seem to sit around at night too... but the crypts typically work best for me since i am collecting cores anyway so I am not going out of my way to find them.

I'd like to see more in the crypts personally.