The Knight bus (a large purple bus seen in the third movie - Prisoner of Azkaban) is not to be found at WWOHP. It was shipped over from the UK for the premiere, but neither that bus nor a replica of it is present in the island at this time.
I saw a post on another site where the bus had been broken down (the second story removed) and being prepped at a port in Georgia, I think, for the trip back to the UK.