Who runs the joke shop in Harry Potter?
Twin brothers Fred and George Weasley run the joke shop in Diagnon Alley called Weasleys' Wizard Wheezes. They opened the shop when Harry Potter gives them a loan from his winnings at the Tri-Wizard Tournament in the fourth book, Harry Potter and the Goblet of Fire. It is during the fifth book, Harry Potter and the Order of the Phoenix, that the brothers leave Hogwarts when Dumbledore is removed as headmaster and replaced by Dolores Umbridge who turns the school into a strict boarding school with many new rules. Fred and George use many of their "products" on her and sell them to the students to use them on Umbridge as well.
The best place to find these are in the Harry Potter world in Universal Studios Islands of Adventure. They have the candy shop, wand shop, joke shop you name it. If you can't go inside the park, outside of it is "Citywalk". One of the stores there sell almost everything that is available in Harry Potter world.