Hi there. I know you said you're looking to buy, but don't forget if you're in Auckland, Auckland Libraries has access to Afrikaans books. There's both children's and adults, fiction and non-fiction:
http://www.aucklandlibraries.govt.nz...languages.aspx You don't have to travel to those libraries, you can request them to come to your local library for you.
Auckland Libraries also has access to Overdrive and Zinio where you can get Afrikaans books and magazines.
A quick search of the Wellington City Libraries and Christchurch City Libraries catalogues also shows there are books in Afrikaans in both cities.