Thanks, great question!
The top source material for faery faith is recordings of testimony from faery faith practitioners, their songs, prayers and folk tales. Thankfully as faery faith is a living tradition we have a lot of this. People took a real interest in recording it during what is known as the Celtic Twilight, when folklore as a academic field was really gaining legitimacy and rural life was making way for more city life. However, these beliefs did not die out entirely and even a number of city people hold them too.
(sorry there aren’t a lot of good websites)