Don't know about downloadable resources,but the "Breton tune and dance book"by Dave Shepherd,(Dragonfly Music,10 Gibson street,Newbiggin by the Sea,Northumberland NE64 6PE),is a passable book to start with.I find the dance section more appealing than the sheet music,but some of the most popular breton tunes are there.

there is also the Breizh ar Braziz (hope the spelling is correct),the book in which apparently a good portion of the Breton music heritage was gathered(the breton version of the Child's Ballads),and I'm sure you can order it via internet,although definitely not a single page of it was published on internet(I enquiried around a while ago.)
there are also a couple of threads to refresh on the subject,and if you are looking for a crash view of the breton folk scene the 'Planet Bretagne' double CD set is a good one.I'm sure someone here can help me suggesting the label of the 'Planet ...' series of compilations.