I would start looking around on the rental car sites now. I believe that if you wait until you get there, you will be paying premium dollars. If you book through either
Alamo or National you can rent at either the CCC or at the Dolphin. If you go with the CCC, you would need to call them the day before you are due to pick up the car, they will tell you a time that they will send a shuttle to your resort for you. Then, you reverse that when you return the car.
If you rent from the Dolphin, you can just walk over there on a day you are visiting Epcot. I did just that last month. My pick-up time was at 3:00 (the desk closes around 4ish), I did the paperwork, they gave me the keys. We returned to Epcot for the rest of the day, after IllumiNations we walked back over to the Dolphin, handed the valet our keys, he drove the car around and off we went. It was really nice to not have to worry about parking the car for the day, or having to pay for it to be parked. Then, reverse it when you are done.
It was the first time renting for me and it was incredibly easy!!!!