The Internet Sacred Text Archives is a source of sources per say. You will find sacred texts for any topic including a listing for Tolkien. The listing is made up of books which inspired the Professor to write his Middle-earth sagas. The Volsung Saga and The Mabinogion are but a few of the sources Tolkien used.
A truly interesting site on it's own and helpful for those doing research on the Professor.
The Internet Sacred Text Archives