Your first move should be to research South Africa and find out what's available beforehand. Check out forums about living in South Africa, online articles about the food industry in South Africa, etc.
I recently moved to Turkey, where I'll be living for a year Teaching English. I brought a few things along with me: coconut oil, sea salt and some supplements. The coconut oil has been a life-saver, as it's been impossible to find healthy animal fats while in Turkey. All of the beef and lamb in Turkey is corn-fed; there's no grass-fed meat.
As such, I've had to adjust my diet from what I ate in the states. My main protein sources are chicken and fish (mostly wild-caught sardines), along with some squid, and a few different types of farmed fish. Every now and then I'll splurge and buy some ground beef (which costs upwards of 30 Lira per kilo, about $7.50 per pound), but, for the most part, I stick to lean meats cooked in healthy oils.
I also find myself eating a lot more fruits and vegetables, as farmer markets are all over the city, and are absolutely humongous. Fruits and vegetables are cheap, healthy, and easy to incorporate in every type of meal.
Once you get to South Africa it may take quite a bit of legwork to find the best places to source your food. Take a walk around your neighborhood (if you're living in a big city) and become familiarized with the different markets, grocery stores, butchers, etc. In South Africa, you'll have the additional advantage of everyone speaking your language, making it easy to find out how your meat was raised and where it came from.
Hopefully this helps a bit! Have fun volunteering abroad!