OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Ymchwiliad dilynol i dlodi plant : ai addysg yw'r ateb? : crynodeb ac argymellion [sic] mewn Cymraeg clir. Published by Cynulliad Cenedlaethol Cymru in 2011. Publication and catalogue information, links to buy online and reader comments.