OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Almanac y teulu : llyfr pen-blwydd gyda phwt o'r almanac ar gyfer pob dydd o'r flwyddyn. ISBN 0863811647. Published by Gwasg Carreg Gwalch in 1990. Publication and catalogue information, links to buy online and reader comments.