Try searching carrentalsavers.com and autoslash.com. They pretty much consistently have the best prices for rental cars. Just make sure to rent from one of the major agencies at MCO. Off-airport locations are significantly cheaper (they don't have to pay the fees, etc as the in-terminal agencies) but people report having huge issues with these companies. First you have to take a shuttle to any of these agencies (except Sixt- don't know much about them but they are "big" in Europe), which adds time. The off-airport agencies are known for trying to upsell you, HARD, lying about needing to take their insurance, not having enough cars (so you wait for hours to get a car), etc. I can get 12 cents out of a dime when I travel, but I will NOT rent from any of these off-airport agencies, even though it could save me over $100 on the rental.
There are some things to keep in mind with all car rental companies. You don't HAVE to take the rental agency's car insurance, although they will ask (and maybe pressure you) about this. Most personal car insurance policies will cover rental cars, and many credit cards provide secondary insurance. Check your policy and if you have coverage, just say "nope, my personal policy covers it" and they'll move on. It's also not a bargain to pre-purchase a tank of gas, unless you are willing to cruise into the parking garage on fumes at the end of the rental period. I fill up at the Speedway (?) across from Disney Springs and have never had the gas gauge indicator move off of "F" by the time I get to MCO. If you are worried, there is a gas station at the south entrance to MCO that will have not great, but OK prices, for gas. Just don't pre-purchase a tank of gas from the rental company; the only benefit to pre-purchasing gas is to the rental agencies!!