I would call Disney directly. Some people like the Disney-specific travel agencies which I've never used. Other than those why get a third party involved which, at best, will do everything you can do yourself? I've read about too many situations where the online travel agents (Expedia, Travelocity) make mistakes and it's a nightmare. No place is going to get you big discounts on tickets, at least I've never seen any.
So I would recommend doing it yourself.
Bob