This is the type of thing that can vary from city to city. If you want to buy something, you might contact a store and ask for a personal shopper to help you. They know what people wear to these things in your location. Also your newspaper's society page (probably on Sunday) will show photos of people attending similar events or even the same event from last year. You might try Googling event to find the archives to see what is worn.
As an over-60 who would rather not wear revealing cocktail dresses, I would probably opt for a dressy suit or LBD with a stunning jacket. With either, you may not stand out but should fit in nicely.
