Hi Debbie
I've bought mine from Travelex (I'm off to South Africa on 9 Sept). They usually have better exchange rates than the places like the Post Office, Thomas Cook, M&S etc and do next day delivery, or you can order online and pick up at the airport if you want.
They also do a South African Rand Cash Passport which you load up with Rand and then use as a cashpoint/debit card. Due to the limit on taking 5000 Rand in cash into South Africa (which certainly wont be enough!!), I think this is safer than taking my credit/debit cards.
Hope this helps
Lynne