I must be lucky as hell.

My list is around 70 confirmed (meaning I've had it or I've watched someone pick one up there), and I've only had one mage in the last two years (out of 5-6) unable to find his black. I've had a couple who never found their siennas, but after they added that class-specific shield spell, I quit trying hard to find siennas.

I know of a couple explore areas locations, as well as a handful that I'd delete over because they're too hard for anything but a Lich.

I'd be interested to know which one is obtainable only "once every few RL hours". I haven't come across this one or anyone who had it (and knew so).