I've usually simply researched via google maps (zoom into the area where you try to find a campsite, then search for "camping". Not everything shown as a camping really is one, sometimes it's a parking area for camper vans only, or a vacation resort with small huts. But that's easy to find out. Simple google search will mainly show you the big expensive vacation resorts. Via google maps you find more of the smaller campsites.
What is also sometimes helpful is to have a look at the websites of the mairie of the towns. The municipal campgrounds are often tiny, with little luxury, and don't always have their own website - so in a simple google search you might not find info about it. Since they're run by the town, info can be found on the town's website, usually in the "tourism" section. The municipal campings is what I prefer. More quiet and less expensive than the resort-like luxury ones.
For that kind of campsites booking is usually not needed, at least not for a small tent. If there's no check-in, someone will come and collect the money in the evening.
I've got a long list of campsites through France, but I guess you'll stay much further north? (I entered France from Cologne-Trier-Luxemburg and then went via Cluny and Le Puy).
Happy planning and buen Camino!