Nothingt should limit you not to buy ethereum in any of the African countries. South Africa is one of the popular African country and one of the first countries that started using bitcoin and other cryptocurrency as we even have some project that originated from south African.  https://remitano.com/az is one of the many websites you can buy ethereum in south Africa and it is not only in South Africa but any of the Africa country. You can buy direct through bank transfer and this is the most secure exchange I have came across in Africa and I have used.