Here is a list of Junkboxes you can get with Pick Pocket. You can train your lockpicking skill with these too, however in this Lockpicking guide, I will mostly use Footlockers and Doors.
1 – 100
These chests below respawn really quickly. Skill up from 1 to 100 will be really easy.
[Buccaneer’s Strongbox] – These chests located on a ship near Rachet in The Barrens.
[Burial Chest] – Blood Elves can try this one
[Practice Lockboxes] – Redridge Mountains
100 – 170
The Rogue poisons quests for Horde and Alliance are the best and easiest way to level up to 170. As of patch 3.3 you can no longer gain skill ups If you’ve already finished the quest.
[Gallywix’s Lockbox] – The Barrens
[Duskwood Chest] – Westfall
If you already finished the quest, go to Ashenvale – Zoram Strand, you will find [Waterlogged Footlocker] there, which you can use to level up to 150 or so. After 150 go to Hillsbard Foothills – Durnholde Keep and lockpick [Battered Footlockers] up to 175.
170 – 225
Now you have to go to Badlands – Angor Fortress. You will find two kind of footlockers there, you will need a lockpicking skill of 175 to open the [Dentened Footlocker].
[Battered Footlocker] – Angor Fortress – Upstairs
[Dented Footlocker] – Angor Fortress – Downstairs
Don’t bother trying to Pick Pocket the mobs, the boxes you will find are already grey to you.
225 – 250
Go to the middle Searing Gorge, it’s near Badlands and go inside the caves, the Footlockers will be there. Now you can try to Pick Pocket mobs too, because the [Sturdy Junkbox] you will find turns grey only at 275.
[Dented Footlocker] – There are two kind of Footlockers with the same name, one of them turns green at 250, the other turns green at 275.
250 – 300
Go to Tyr’s Hand in Eastern Plaguelands. Pickpocket the mobs and find all of the [Scarlet Footlockers]. In the time it takes to take a full circle of Tyr’s Hand, the starting point will be pickpocketable and have boxes respawned.
300 – 325
[Wicker Chest] – Zangarmash
You can also Pick Pocket the mobs here, because they drop [Strong Junkbox].
325 – 385
[Dented Footlocker] – Nagrand
Same as above, Pick Pocket mobs.
385 – 400
[Scarlet Onslaught Trunk]
Or you can Pickpocket any Humanoid mobs at Northrend to get [Reinforced Junkboxes].