To unlock these special items, you must beat the game and they will appear in your inventory when you load it up again.

Once you get your Thief Suit, go to the castle and find the one of the mimic. The best one is in a room in the hallway where the monkey was. Keep going in and out opening the mimic to get more money. Make sure not to kill the mimic. Just hit him a few times until he runs away so that you can open him.