Except during the summer period when the French start their vacations from the end of July to the end of august you could should not have much trouble finding your accommodation. However there is no harm in booking advance and doing a bit of research before you arrive in France. Be aware that Paris is the exception to the rule in many ways and certainly in terms of hotel prices and availabilities. Have a look at the Kelkoo search and compare prices of the cheapest hotels available at the time being. Outside Paris you might find really cheap hotels from 40 Euro per night upwards. In the center of Paris and also Nice you might look at starting from 80 Euro for a decent mid-range hotel accommodation.