Some of the ticket brokers have them occassionally, but it doesn't sound like any one is offering them at this time. Alamo car rental used to offer them through a special phone number, but you have to have an Alamo reservation number to get them. You can make a reservation, get your tickets, and then cancel.
I would probably try to get a length of stay pass from the onsite hotel. I've read that people can make two reservations to get an Ultimate Park Hopper.