The Stolen Weapons are part of a quest on Fort Finch, and a lot of players have reported not being able to find it even after searching the Island for a long time. Anyway, it's pretty easy to find if you know where it is, and if you rather want to search for yourself, then click out of this post now, because there are spoilers ahead.
In order to get the Stolen Weapons needed for the quest, go to the portal at the top of the level. Now enter the building right behind the guildmaster (that is right next to the portal), and ascent the stairs. There is an NPC at the top of the stairs who is really nervous about the weapons, and that's not strange, because there is also a chest right next to him. Loot this chest and get the Stolen Weapons.
Now head back to the cells to deliver them, and you're good to go.