I am visiting south africaat the moment, i just took the ipad along to the local vodacom shop. They do sell sims in stores but you really need to go the either a vodacom or ment cellular shop to get it there. There is cellc also but they arent as easy to find, and since its better to have one you see everywhere i would recommend vodacom or mtn, i chose vodacom. They have little cellular shops in every big mall and most smaller residential mall type buildingns.
A sim itself costs basically nothing, but they have to convert a regular sim into a microsim, and they charged me R110 for that. Then they have to register your number with RICA. Every phone or3g number has to be registered to work. To register you will have to show your identification (passport) plus evidence of where you are staying in south africa, it has to have a printed proof of street address. I stay with my folks when here so i took their power bill. If you are in a hotel just find out from the hotel what they cangive you.
Once you have donethat, you haveto buy a data bundle. As it can take a few hours for your number to be active once sometimew cant load the data immediately. I went on dayy one, did all the above and then went the next morningto load ondata.
They are open on sundays, so that helps.
I am here for 6weeks and a loads of iinternet user so i got a 3 gigabytes bundle and it cost R520 for the bundle.
Good luck, ask if you have any more questions i am using my 3g right this minute, from pretoria.