depends on your airline. They have different policies. some will carry it free of charge as a "personnal item" on top of your kid's baggage allowance, some will consider it a second bag. Some will just gate check them as if it were a stroller (but you can't gate check a stroller AND a car seat for the same passenger.
Closest supermarkets where you can buy a car seat are located on US192 or on 535
On US192 you have 2 super target. One East of the property (quite a ride though) and the other one is on the West of the property, past AKL
You also have
Walmart which might have a larger and cheaper selection. It's located on 535, halfway between crossroads and US192
Last resort is Sears at Florida Mall
Taking the Lynx to either of those stores will take you at least one hour one way.
An alternate option, if possible, would be to send the driver, alone, to pick up the car while the rest of the crew enjoys the resort. This driver would then proceed to the store, pick up the seats, and then pick up the rest of the party.
bying from walmart, you may want to buy the seats online in advance and choose "store delivery" so that you're assured to find what you want one in the store.