User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-4563270-20150320133520/@comment-5043481-20150320175548

You might need to do gathering hall quests to unlock it. In 3U I think they were entirely gathering hall dependent. I am at G2 I can send them to those places but I cannot remember when I unlocked this. Sorry :\

Either way, persistant mining in heavens mount is probably the best way to get them.