How do you get next weapon after you buy the polished silver sword from the Blacksmith?
The next weapon from the Blacksmith is the Scythe. It costs 5,000,000 candies and doesn't even appear until you have a large number of candies.
The next weapon you're likely to find is either the troll's bludgeon from defeating the Troll at the Bridge, or the Monkey Wizard's Staff from the Monkey Wizard in the cave.
For more information: http://candybox2.gamepedia.com/Weapon