Rafing in RSA
Rafting in SA is pretty hit or miss. It's generally rain dependent.
The most common runs are on the Orange River in the Richtersveld, the Doring River in the Western Cape, and a variety of rivers on near the southern coast. Durban is a decent place to base out of which gives access (with some driving) to the Umgeni, the Blyde, the Tuglea and the Buffalo further up into Zululand.
There are also some smaller runs up in kwaZulu like the Pongolo that are worth exploring. You'll need some time and a reliable 4wd shuttle vehicle if you want the flexibility to get out and explore on your own.
The best time to raft is usually between November and May.
Graeme Addison and the folks at the Southern African Rivers Association might be a good resource. Graeme has been involved in the RSA river scene since it's earliest days.
I've spent a fair bit of time on the rivers down there so feel free to hit me up if I can be of any assistance.