Car rental is available in towns all over France. The major car rental companies (Avis, Budget, Thrifty, Hertz, etc.) have desks in the airports. The companies at the Nice Cote d'Azur airport and the Marseille Provnce airport are: Ada, Avis, Budget/Milton SAS, Europcar, Hertz, National Citer/Alamo, Sixt.
Booking and pre-paying your rental before you start your trip (at least from North America) is the cheapest solution, allowing you to save up to 50 percent on the cost of renting when you get there.