WoWDB

 Graccu's Mince Meat Fruitcake is contained in  Smokywood Pastures Sampler. It is mailed to your character after completing Treats for Greatfather Winter / Treats for Great-father Winter.

Edit: From BlizzardCS (verified Blizzard Customer Service account) on Wowhead -

Hey guys! We have some awesome news for this year's Winter Veil. We recently received a letter from Graccu himself, explaining us some changes he decided to introduce this year to make everyone's life easier. We thought you would enjoy the read:

"Graccu sad of making people sad. Graccu want people happy. Graccu want people enjoy Fruitcake. Graccu make more this year. Graccu now extend distribution chain to include retail market through Smokeywood Pastures. Graccu put Fruitcake in Stolen Presents too. Graccu want people happy. Graccu out."

So this year, for all your joy and happiness,  Graccu's Mince Meat Fruitcake will be much easier to obtain. You can buy it from Kaymard Copperpinch or Macey Jinglepocket for 99 gold 99 copper. Additionally, you'll also be able to obtain one Graccu's Minced Meat Fruitcake each day from  Stolen Present!

Happy Winter Veil to all of you! :)

The color of the winter clothing doesn't matter. You will get the achievement as long as you are wearing a hat, clothes, and boots.

The clothing and the boots may be crafted with Tailoring and Leatherworking, respectively. Horde tailors will have to go out of their way (read: brutally die in Ironforge, slightly less brutally in The Exodar) to acquire the pattern for Alliance's red clothing if they want it; Alliance has it much easier since the vendor in Shattrath sells the pattern for Horde's green clothing.

The hats are the difficult part of the achievement because they are Bind on Pickup. It is possible to loot a hat as a world drop from humanoids but the chance is very, very low. You are much better off farming certain dungeon bosses. Keep in mind that the hats are 100% drop rate only in Heroic difficulty. The drop rate in Normal difficulty is dramatically lower.

Green Winter Hat

• Wrath of the Lich King dungeons
• The Oculus - Mage-Lord Urom
• Cataclysm dungeons
• Grim Batol - Drahga Shadowburner
• Shadowfang Keep - Lord Godfrey

Red Winter Hat

• Wrath of the Lich King dungeons
• Ahn'Kahet: The Old Kingdom - Jedoga Shadowseeker
• The Nexus - Grand Magus Telestra
• Cataclysm dungeons
• Blackrock Caverns - Corla, Herald of Twilight
• The Deadmines - Admiral Ripsnarl
• The Stonecore - High Priestess Azil