Iwhich part of Brittany are you going to (I have some recommendations for the stretch between St Malo and Roscoff - although I might need to rack my brains a bit). Also when are you going some places don't open until late May or even June?
Camping municipals are usually a good bet - although there are plenty of decent, and inexpensive, private sites as well. I paid between 5 euros and 12 euros with an average of 7.5 euros more or less. IIRC the regional tourist office website has lots of info on campsites. Google Maps is also pretty reliable - most sites have a website. If you speak French, the two Guides Routard were very useful.