Dropped from Berry Grub near Passhow Marshlands zone. Black Chest, 3rd guess. During Mystery Box: Windy, Mystery Box: Bastok, and Ease of Exploration superkupowers.
Got these to drop from a casket in Rolanberry Fields. Dropped from a crawler or a goblin I believe. These are awesome for dnc soloing (as I was soloing on dnc at the time when they dropped). Equipped them over my Leaping Boots, but will def put the Leaping Boots back on for exp.