Argggggg; that was the place (I think) where I Had to find 24 leaves. It took forever to find the last one.
Its in Blackreach, which is in Mzinchaleft, which is on your map.
I found it by wandering around. Didnt realize it was a main quest thing or I would have avoided it. But at least that will speed things up when I finally go back to the main quest.
http://elderscrolls.wikia.com/wiki/Mzinchaleft