Venture Into the Bizzare (1976)

Two young girls in New York's Greenwich Village own a plaster-casting shop. One day a well-known Hollywood actor walks into their shop and asks if they can make a plaster replica of his penis. When he later brings a friend of his, also a well-known actor, in to get his "member" replicated, the girls hatch an ingenious plan to make a lot of money by making replicas of stars' penises, then blackmailing them. However, things don't turn out exactly the way they planned.