Good Morning Debbieandnorris,
To purchase South African Rand I would suggest that you first look at what the current exchange rate is on XE.com. From this, I would then shop around for the best deals (e.g. in stores, at the airport, post office).
I know that when Julia went abroad to South Africa she actually exchanged her currency overseas, so perhaps this is something you could also do?
I hope this helps.
Kindest Regards,
Amy