You are going to have to try the stores closest to the parks. Also Sam's Club. Get the zip code and search proximity and call the customer desk. They are usually quite helpful. Also try BJ's.

These things come and go, but I have definitely seen both Disney and Universal tickets/passes at both Sam's and Costco in Orlando in prior years; you may not have a lot of selection, though.

The Harry Potter rides/Hogwarts/Hogsmeade are indeed amazing. Get there first thing.