From a general agency perspective Hertz has the best fleet for "non" standard rental cars with their Adrenaline (formerly Fun Collection) and Prestige collections. Unfortunately the Corvettes are pretty much gone from the fleets, although they do still exist in small pockets, but are not reservable, only upgradeable.
If you want a true Exotic (ie. Ferrari, Lamborghini, etc.) you're going to need to look localized. I know in NYC, Boston and Miami there are a lot of choices in this arena, Orlando not so much and in smaller cities, very, very limited.