National Car Rental has an office located at the Car Care Center outside the Magic Kingdom and at the Dolphin Resort. Shuttle service to the Car Care Center is available from all resorts and must be reserved at least one hour in advance. National is open seven days/week from 7:30 am to 6:30 pm. The phone number is 407-824-3470 or 800-227-7368.
I always rent from National at the Car Care Center. I've never waited very long for the shuttle (maybe 15 minutes?). The shuttle also will take you back to your resort after you drop the car back off. I usually drop my car off the evening before we leave after we are done in the parks (we usually exit the park around 4 or 5, so we can drop the car, eat, and pack).
Rented from Alamo at Car Care Center in NOV and it could not have been easier. Shuttle picked me up from OKW to pick up the car and then when we returned the car, the shuttle driver took our wholte group (6) to EPCOT which is where we wanted to go!