In Mooshu, they are a rare drop of Black Lotus. There is also a Transmute Black Pearl spell available. A quick search of this forum will bring up TONS of posts and tips on this very subject. :)
Jump realms to collect Black Lotus and the very rare Black Pearl in MooShu (Find a place where the Black Lotus appears in two places - usually at the end of the road near a building where you see a cart on one side and a rock on the other)
Try the Bazaar (very rare find here)
It will take time to find or craft this item.
Also do a search of "Black Pearl" in this forum, there may be more ideas that have already been discussed in other topics.
The best place to find Black Pearl is in the tree of life cause the tree has allot of black lotus early today i got 6 black lotus and 3 black pearl while working on Everlating Forest in th Tree Of Life. :D