Alamo and National. Both are at the Car Care Center near MK. There are also locations at the Swan and Dolphin (one is at one, the other at the other).
CCC has a shuttle that will pick you up and drop you off just about wherever you need to be. Resorts, parks (though if you are/need to be at MK it'll actually be at TTC), WWofS. Call them *at least* an hour before you need to be picked up, and don't give yourself too short a time when dropping the car off because you put your name and destination on a list, and they dispatch shuttles as needed and possible.
I've only dropped off at Swolphin and just walked over to Epcot, so I don't know exactly how that works.
Anyway, just go to Alamo or National's sites and price it out. Sometimes using those locations is a great deal (their taxes/fees tend to be lower than getting it/dropping it off at the airport), and sometimes it's not (the daily rates are sometimes much higher than the daily rates at the airport). And once you find a rate you can live with, book it in a way where it can be changed/cancelled. Then keep on checking, because as the time gets closer, rates CAN go down. Not always, but it's possible, so keep on checking.