As with airlines the cheaper rates go first so while you may still get a car you won't get the best deal from the major companies. There are a number of smaller local companies that can be good value but I would always recommend booking in advance. Also be aware that some of the airport outlets can run our of cars during the peak season. By booking with Hertz for example you can secure a good rate with no obligation so if you find a cheaper car on arrival you can cancel initial reservation.