Hello
So I've never bothered with Darkmoon faire till recently when I came across a quest that grants you 250 rep for a 36xGreen Firework turn-in and a Harvest Festival NPC sells you that very item for the quest very cheap. Green Fireworks is the Darkmoon repeatable and the Harvet Festival NPC outside of IF/Ogrimmar sells the fireworks item which otherwise would be a bit expensive to buy off the AH during this period.
Note that it stacks up only till 5 so fill up !
Have fun !...