Hi
It would depend on which countries in Africa you intend to visit. Some countries require you to have a carnet.
I have just returned from a trip round Southern Africa and you can buy third party insurance at the border - in fact YOU HAVE TO.
The countries where we had to buy insurance at the border were Zambia and Zimbabwe. On entry into Botswana we also paid insurance but it covered us for travel in Namibia, South Africa, and Swaziland .
Hope this helps. Let me know if you want any specifics about Southern Africa.