"Harp" is an Item in the Box 1 deck that works similar to Petrificus, but applies only to Creatures. So either this item just hasn't come off the Hogwarts deck for you, or you may want to check the card listing and make sure it isn't missing from your copy of the game.
Harp applies only to Creatures, and Petrificus applies only to Villians. An important rule distinction with many of the cards from the base game.
Also, with as bloated as the Hogwarts deck gets with all of the cards from the base game plus the new cards from the expansion, it is quite likely that you could go many games without seeing a particular card come up into the market, especially if there is only one copy of it.