This quest can be acquired from Swordsmith Fink, who can be found at his forge behind the weaponsmith in Sasau. He is interested in recovering five broken fragments of a legendary sword, which is said to have belonged to the Queen of Sheba. Most of the pieces have been sold to blacksmiths in the realm; they like to keep them as a good luck charm.
Sasau - Blacksmith Zach
You should begin by speaking with Zach, the blacksmith who works at his forge just south of the monastery, as seen on the map below.
Zach will inform you that he does own a piece of the sword, but that he does not want to sell it. You will therefore have to pickpocket him. This will be very difficult to do during the day, as his son Vitus is almost always by his side. Instead, you should wait until after midnight, at which point you can go to his house and find him sleeping, the location of which is shown below.
Ledetchko - Blacksmith
The second piece is much easier to acquire. Simply visit the blacksmith in Ledetchko and ask him about whether or not he has a fragment. He will tell you that he does, and that he regrets ever buying it. You can purchase it from him for 200 Groschen.
Rattay - Pickman or Blacksmith
If you have successfully completed the quest 'Aquarius' and assigned Pickman to the job of water carrier, the this fragment will be easy to acquire. Simply find Pickman (he will be walking up and down the slope through the south west entrance to Rattay during the day) and ask him about the reward he promised. He will then provide you with his fragment of the sword of Sheba.
If you have not yet completed the quest 'Aquarius' and do not plan to, or if you did not assign Pickman the job of water carrier, or if you did not ask for a reward when you spoke with him about the job, you will not be able to get the fragment from him. Instead, it will be in the possession of the Rattay blacksmith. The blacksmith stores it on top of a shelf in the back room of his house.
Note: It has been noted that in some cases the blacksmith does not possess the sword fragment even when you have not met the requirements for receiving it as a reward for Pickman. In this case you can find Pickman and either knock him out to find it on his body, or pickpocket him.
Rattay - Weaponsmith
You can find another fragment of the sword in the weaponsmith's house. This will be very difficult to infiltrate during the day, so wait until after midnight before you attempt it. Remove any items with high noise ratings before you begin, and take not of guard patrols so that you can strike at the most opportune time. You should pick the lock of the door to the right of the forge where the weaponsmith works. As you enter the house, go up the stairs directly in front of you. You will find the chest with the sword fragment in immediately to you left at the top of thie stairs. Be sure to remain crouched to avoid creating any unnecessary noise.
Talmberg - Blacksmith
This sword fragment can be found in the blacksmith's house in Talmberg. The house is not marked on the map, but you can see the location in the screenshot below.
As you enter the house from the front door, you will find the room with the sword fragment through the second door to your left. The sword fragment will be placed on top of a barrel in the corner.
Once you have collected all of the sword fragments, take them back to Swordsmith Fink in Sasau. You should now wait 24 hours to give him enough time to reforge the sword. When you speak to him again, the quest will complete and you will have the option to purchase the sword.