WoWDB

IMPORTANT: This achievement is not part of the meta-achievement  Tricks and Treats of Azeroth.

For the "Tricks and Treats..." series of achievements, I recommend installing the add-on Candy Buckets (Hallow's End). It adds small candy bucket icons to your world map, and even knows and remembers buckets you've visited! It has TomTom (and TomTomLite for those users) integration, too. Spiffy!

edit: compiled a list in the order in which i grabbed buckets. character was level 87 with flying mount.

TRICKS AND TREATS OF NORTHREND
Dalaran, Sunreaver's Sanctuary - 67,30 - enter The Filthy Animal; the bucket is on the left wall.
Dalaran, The Legerdemain Lounge - 48,41 - on the bottom floor and across from the bar.
Dalaran, The Underbelly - 38,60 - go into the Dalaran sewers and head to Cantrips and Crows. Run up to where the Silver Covenant Agent is and then run left.
Dragonblight, Agmar's Hammer - 38,46.5 - inside the building with a mailbox nearby.
Dragonblight, Moa'ki Harbor - 48,74.5 - inside the building opposite the flightmaster.
Dragonblight, Wyrmrest Accord - 60,53.5 - on the bottom floor, next to the reagent supplier.
Dragonblight, Venomspite - 77,63 - inside the inn and past the innkeeper. look for the building with a mailbox.
Grizzly Hills, Conquest Hold - 21,65 - inside the building with a mailbox. run in and turn left.
Howling Fjord, Kamagua - 25.5,60 - inside the building with a mailbox (does not look like your average mailbox). run down the ramp and you'll see the bucket.
Howling Fjord, New Agamand - 52,66 - inside the building with a mailbox. bucket will be on your right.
Howling Fjord, Vengeance Landing - 79,30.5 - inside building (Vengeance Landing Inn) with a mailbox and flightmaster nearby.
Howling Fjord, Camp Winterhoof - 49.5,11 - inside the inn. just look for the mailbox.
Grizzly Hills, Camp Oneqwah - 65.5,47 - facing the flightmaster, it's in the building to the right of the flightmaster.
Zul'Drak, Zim'Torga - 59,57 - in front of the statue.
Zul'Drak, The Argent Stand - 41,66 - near the mailbox and right next to the Apple Bob tub.
The Storm Peaks, Camp Tunka'lo - 67.5,50.5 - inside the building at the top of the peak. forge and mailbox nearby.
The Storm Peaks, Bouldercrag's Refuge - 31,37 - run inside and look left. if you can't find Bouldercrag's Refuge, look for Dolomite Giants and mole machines.
The Storm Peaks, Grom'arsh Crash-Site - 37,49.5 - inside the crashed zeppelin.
The Storm Peaks, K3 - 41,86 - inside building, left of innkeeper.
Sholazar Basin, Nesingwary Base Camp - 26.5,59 - under the open-air tent, easy to spot
Borean Tundra, Bor'gorok Post - 49.5,10 - inside the only building.
Borean Tundra, Taunka'le Village - 76.5,37.5 - inside largest building.
Borean Tundra, Unu'pe - 78.5,49 - inside building with innkeeper.
Borean Tundra, Warsong Hold- 41.5,54.5 - go up main entrance stairs part-way, then descend steps leading down. go straight, pumpkin is obvious.

*roughly circuitous starting from dalaran
*if you aren't running Candy Bucket (Hallow's End) addon then enable Track Townsfolk -> Mailbox from the menu button near your minimap. it will be very helpful.
*unless otherwise specified, assume all buildings are inns with mailboxes.
*all coordinates rounded to the closest half point.
*and they're zone-specific.