If you need a car for other reasons, this may not help, but a lot of people use wegoshop.com. I have not personally used them, but every comment I've heard about them has been a rave review.
Wegoshop will take your grocery list, shop just about anywhere you want - including Costco and other discount clubs - and bring the groceries to you for a VERY nominal fee.
Another option is Enterprise Car Rental. They will usually deliver a car anywhere you want.
You might want to go to a site like Expedia and do a search for a car there. Instead of the airport, say you want to pick it up at WDW and see what kind of results you get.
Just one hint - when you get the list, sort it by price. I did that yesterday, looking for a car in St. Thomas and found that the cheapest car was a midsized car from Thrifty...which was $1 per day CHEAPER than Thrifty's smallest car. Rental car prices make no sense, and I see strange things like that all the time.
The other thing you should do in comparing is check the model of the car. You will find that some companies consider a particular model a full-size, while another company calls exactly the same car a mid-size and rents it for less.