I know you can get them from Mad King Chests, but I've heard that you can get them from other places as well.
To get a Mad King Chest, just give the Mystic Forge 1 Candy Corn, 6 Mystery Tonics, and 2 boosts of any kind (like the ones you get from the Black Lion Chests, they must be placed separately in different boxes). I think the drop rate is low though. http://wiki.guildwars2.com/wiki/Mad_King_Chest
Actually, it seems like you can get them from regular old Black Lion Chests during the Halloween event. I think the Mad King Chests are intended to drop Halloween stuff after the event is over though. They don't require keys though, which is nice.
