OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Y defnydd o farddoniaeth Gymraeg mewn gwersi addysg grefyddol by Ffuon Puw. ISBN 1853570850. Published by Canolfan Genedlaethol Addysg Grefyddol in 1996. Publication and catalogue information, links to buy online and reader comments.