- Thieves' Tools
- Binds when picked up
- Unique
- Requires Level 15
- Item Level 15
Thieves' Tools were required for rogues to pick locks prior to patch 3.0.2.
Source
This item was sold by Shady Dealers, Poison vendors, and many other goods vendors.
However, there was no need to buy Thieves' Tools, they are also a quest gift from the following quests:
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Although no longer needed to pick locks the tools are still provided by the quests.
Patch changes
Patch 3.0.2 (2008-10-14): 
[Pick Lock] and 
[Disarm Trap] no longer require Thieves' Tools.
Notes
If your character already had Thieves' Tools prior to the patch, the Thieves' Tools were not removed from your inventory. They are now junk (but quality was not reduced), but might be kept as roleplay items.