I think it's random. Just keep on rumbling and u will find one
eventually. I reccomend sending your pokemon for 12 hour straight
to increase the chance of getting items(this includes explorer
bags)
. . . . . . . . . . . . . . IN SEARCH OF MY LONG LOST
COUSIN, TRASHBIN . . . . . . . . . . . . . . .
Please contact me if you found anyone playing with grimer,
trubbish, and koffing since it's likely TrashBin.