White Pale works similar to the every random bosses with Reward Container system. It spawns between 15 to 28 day after it was killed.
You can find White Pale west from Liberty Bay:
West from Darashia:
South from Edron:
From my own experience I can say that chance to drop one of these items is quite big. I would say that both of rare items are "uncommon", definitely not so "rare".