http://www.bretagne-sud.com/ has a very short list of Fest Noz (under which it includes the huge Lorient!)as well as a listing for fetes. That just for the south of Brittany of course; which part are you going to?

There may also be listings in the newspaper http://www.bretagne-online.com/

another general site which you can search for listings is http://www.breizhoo.fr
but a lot of small local events which are so enjoyable would not be listed on the web