Give ebay a try here is the UK site searched under children’s books they then list age groups.
http://search.ebay.co.uk/search/search.dll?sofocus=bs&sbrftog=1&dfsp=1&catref=C6&from=R40&satitle=children+books&sacat=-1%26catref%3DC6&sargn=-1%26saslc%3D3&sadis=200&fpos=TN23+7SH&sabfmts=1&saobfmts=insif&ftrt=1&ftrv=1&saprclo=&saprchi=&fsop=1&fsoo=1
That’s perfect! I’ll have to look at what the US site has available...