As you may guess, Disney does not provide transportation to US/IOA. Your only real options are to take a taxi (maybe some one who's done this can post a rough price estimate for you), or rent a car.
Disney does have a place to rent a car from on the property. You could rent it for the two days you plan to leave WDW property. I cannot recall where exactly on WDW property you can rent a car from (I think you can rent one from any of the deluxe hotels) but I'm sure if you call 407-824-4321 (gen info) they will let you know.
Good luck!