The items that you can find in haunted houses in Festival Plaza are Sacred Ash, Bottle Caps, PP Ups, Max Revives, Max Elixrs, Heart Scales, Patterned Beans, Festival Tickets, HP Ups, Irons, Calciums, Proteins, Carbos, Zincs, Full Heals, Hyper Potions, Ultra Balls, and Hard Stones. As for exact chances, they are not stated, but the items are listed from rare to common (from top to bottom) in the Bulbapedia article linked below.
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