No company provides them free, unless you happen to rent a Daimler-Chrysler model with built-in booster seats. The usual charge is about $10/day + tax. Even if you reserve ahead of time, companies will *NOT* guarantee that a seat will be available when you arrive.
Most commonly, the seat provided will be a Cosco Touriva 5-pt. convertible infant/toddler seat. When in excellent condition, the Touriva is a perfectly good seat (not a Britax, of course, but a safe seat.) Unfortunately, the condition of rental company carseats at MCO tends to run the gamut from "slightly scruffy" to "just about trashed".
My advice is to bring your own. (If you have a really large/heavy model, then buy a Touriva 5-pt. of your own for travel; it will only set you back about $40.) If your child weighs less than 40 lbs, use your carseat on the aircraft, too; it will be safer for your child and do wonders for your sanity.