Have you tried looking at the Celtic section of the Internet Sacred Text Archive?. You could also try The Mammoth Book of Celtic Myths and Legends, Peter Berresford Ellis (2003). Finally, there’s the Celtic Literature Collective – Mary Jones, which also has a lot of good materials and resources that are freely available.
Hope that helps.
PS: The forum wouldn’t allow me to post the links, but a websearch should bring up the sites I mentioned without any difficulty.