Thiefbane is always generated in the inventory of One-Eyed Sam (unless wished for before Sam is generated). ThiefBane is typically generated with an enchantment, but the greater prize is the instadeath by decapitation (10% chance) on monsters symbolized by @.
Acquiring ThiefBane Edit
Given that the Black marketeer is level 24 and carrying an ascension kit, it may be best to kill her with some ruse. A scroll of earth or invisibility will fix One-Eyed Sam on her spot, and highly enchanted firearms or poisoned darts work fine for the kill. More suggestions can be found at Black_Market.
However, this is not neccessary to acquire ThiefBane. If you are appropriately fast, armored, brave or stupid, and have the Technique Disarm and are wielding something you are at least skilled in, you can steal Thiefbane. Beware you still risk decapitation. If you try that, you will need an escape plan that works despite the Black_Market's restrictions. One way is to apply (break) a wand of digging, trapping her (away from the door).