You have a few options:

• Your local bank or credit union. If you have enough time to order the currency, this is a decent option. Keep in mind that your bank may need to order the currency and may therefore charge you a fee for that service.
• Airport Kiosks or other exchange houses. This is likely the most expensive option—try to avoid them. However, they are also the easiest option. These kiosks can be found at almost any airport in the world. (Admittedly, we’ve used them when we enter South Africa. We’d exchange $20 or $50 just to quickly get some local currency because we’d need some cash to tip porters, parking attendants, etc.)
• A South African bank, ATM or Money Exchanger. Keep in mind that banks and Money Exchangers have very different rates. You may want to compare prices before choosing one. If you choose to use the ATM, be sure to let your bank know that you plan to use your ATM card in a foreign country.
• Your bank’s ATM network in South Africa. You may be able to get Rands with competitive exchange rates and low banking fees (1-3%). If your bank charges ATM fees, try to withdraw larger amounts at a time. 
• Buy Rands online. These are often the cheapest option because they charge low fees. They will charge a fee to send you the money unless you are able to pick it up from their local offices.