I've arranged a inter rail, with couch surfing as solution to sleep in the night.

 Lets say I have an inconvenient, and I need to reserve a hostel on the same day for the night: is it easy to find a room in August in any city/town in France (except for Paris, I'm not going there) or do I risk the night out :) ?

I'm going to Montpellier, Toulouse, Lyon, Strasbourg... do you think I can easily find a cheap hostel for the night, or are they going to be all busy?