Kaymard Copperpinch in the Valley of Strength, Orgrimmar and Macey Jinglepocket in the Commmons, Ironforge are the questgivers. Here's the steps involved:
You're a Mean One...: The Stolen Treats are looted from The Abominable Greench that spawns around the mountains. He yells "Rawwr!" whenever he appears. The Greench has many different spawn points, so check out this map for a better idea of his locations.
24 Hours after completion of this quest you will receive a Winter Veil Disguise Kit in the mail! If you use this to dance with another snowman in Dalarn, you will earn the A Frosty Shake achievement!