Equip Trick Staff II.
Lug Trick Staff, Treat Staff, and the four variations of pumpkin heads around (as opposed to leaving them in event item storage) so the game doesn't try to give those to you again.
Find NPC who accepts sweets.
Spam trading them the cheapest sweet you can find. It doesn't matter if you've already gotten a costume for that sweet that game day (although I suspect your odds are better if it's your first trade of a sweet in a day). Just spam, and spam, and spam.
It works. Takes a fair bit of spamming mind you, but it's how I got a full set of trick/treat staves and pumpkin heads last year, and it reputedly still works this year. (Variations work for the other items; if you want an HQ item, be sure to equip the corresponding NQ item while trading.)