OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Yng ngolau ffydd : Eglwys y Crwys 1884-2009 : llyfryn y dathlu 125 mlynedd by J. Gwynfor Jones. Published by Eglwys Bresbyteraidd y Crwys in 2009. Publication and catalogue information, links to buy online and reader comments.