If you look at the enforcement of the Btmg with respect to shipments of MHRB (mimosa bark) that question starts to resemble something of a grey area. It becomes clear that growing one's own source plants is the most sensible way forward. From that point, you'd best start searching the forum for information about acacia, phalaris and, possibly, desmanthus. Psychotria is also not completely out of the question, and like all of these plants depends somewhat on your location and living arrangement.