Apparently there’s going to be a secret Mattel-produced MOTUC accessory for the first 1,000 fans who register for Power-Con. No one knows what it is. Odds are it will be a repaint/color variant of an existing accessory, but there’s no guarantee it’s not something new (for example, Spector’s green whip). In any event, Fantastic Plastic Toys is offering the item for $18.50, the cost of a Power-Con ticket and shipping.
It’s worth noting that the text on the Power-Con page says that this accessory will be “first available” at Power-Con, which suggests it will eventually be available through some other means.