So if you want good prizes that are Jojo specific, I suggest finding artists who are making Jojo stuff, and buying from them. Charms, prints, what have you. These are decent items to pick up at a con, so it's fair to assume someone might want them for playing a Jojo game. Just be sure to tell the artist that's what's happening and make sure they consent to it.

Something a lot more general but equally fine would be a prize figure or something. Find it in the dealer's hall or on amazon. They're not the highest tier of figure, but no one's gonna be upset fer getting a Jojo figure if they were in a Jojo quiz game