After the first fight with the Slayers, enter the store and run out to the back and open the door to the toilet. A cut scene will take place and the man inside will be shot. Now enter the room, switch the light on, start the sink, flush the toilet 3x, switch off the sink, switch off the light and return to the shop. In the Corner behind some TVs is a new item. Destroy it and carry the game on as usual until you reach the point where the Slayer will burst through the wall. Instead something else does, kill this and the weapon will be dropped by him.