Best to search and read the sub saharan Africa forum on here. Cut and paste the following into your browser:
In short, you need to buy Comesa as it is quite cheap and recognised by every cop and border etc etc. It's not worth much as insurance goes, only 3rd party, but will keep you out of jail if something happens and save a lot of hassle when people want to check your documents.
Sudan will be the first country where you can buy it I think? Else Ethiopia, in Addis, opposite the bus terminal next to Wim's Holland House. Some recent threads on it in the Africa action.
You state how long you want it for, what countries to cover and on what vehicle. I bought it for one year for a Land Rover for all countries and it was about $150 in Zambia in 2011. It will work in Sudan, Ethiopia, Djibouti, Kenya, Uganda, Rwanda, Burundi, Tanzania, Zambia and Zimbabwe. Not Mozambique even though the certificate might say it will. Botswana and Namibia you buy a permit/insurance at the border (25? Euros for 3 months in Namibia). South Africa has 3rd party insurance included as a tax on fuel.
Worth buying it for the whole year regardless of what you think your plans will be. Then photocopy it 10 times and get the original lamented.