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:
Although no longer needed to pick locks the tools are still provided by the quests.
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.