I've used both Maple leaf Tickets and TicketMania. They're valid ticket brokers, and their ticket prices are lower than what you'd get direct from Disney. Though i haven't used them, Undercover Tourist falls into the same category of "authorized ticket brokers/resellers".
My suggestion would be to shop around for the particular ticket you're looking for. One company will often be cheapest for some tickets, while a different one will be cheaper for others.
-Rob