It may well drop in BoT, but it does also drop (as the Common drop) off the "bees" (Koka'Vor Krovian and Senvar at least) in the tree-complex in the Plane of Storms.
the pattern is imbue diamond spell which is on the first vendor on the right when you walk in the cleric/pally guild in north freeport and the planar kit is a planar jeweling kit bought from the jeweling vendor in the building north of the library in plane of knowledge. This is a no fail combine.