The Thieves' Backpack is a medium backpack that reduces encumbrance by 60% and does not hinder your stealth. It slightly hinders combat skills by -2 points, which is equal to the small backpack, and less than any of the larger standard issue backpacks.
The inventory size of this backpack is 10 by 14 tiles, which is the same as the Medium Backpack.
The Thieves Backpack can be purchased from General Item Trader at World's End settlement for 5000 Cats as well as any of the Shinobi Thieves traders. Additionally, the backpack can be bought at adventure trader stores, though they appear to be sold only occasionally.
It should be noted that the Shinobi Thieves' trader always sells items at a 50% discount, though one is required to be a member of this thieves guild in order to trade with the merchant in the first place.