They drop from Yellowboze Ill Gill (1/1575) and Purplenum Shambertin (1/102,4). Here is a completely legit screenshot of me getting one on Ephinea.
pay no heed to the DESTINY part
sidenote damn it's such a shame ephinea nerfed the shit out of RAcaseal's HP, used to be 2140 at level 200...