Digestive cookies - substitute in South Africa?
You'd probably get away with any kind of sweet hard biscuit. 

I'm not South African, so I can't help you if you want a local biscuit, but looking at Woolworth's online shop suggests you might actually be able to find Digestives, or there's Bourbons, Shortbread, or even Ginger Nuts if you fancy trying something a little different from the recipe.

Note that although the recipe calls them cookies, it's just using the American term for biscuit (even though it seems to be a British website).