Info taken from JN so credit to them:
Locations:
Lampwyck's Lights Fantastic
Giant Jelly
Lunar Temple
Healing Springs
The Crumpetmonger
Bag to buy in bulk would be The Crumpetmonger Trick-or-Treat Bag around the 500k mark .
The bag gives out a stamp but it is fairly hard to get the stamp so currently the stamp prices are hovering around the 2m mark and should rise quickly with time.