Maun in Botswana if you have C 206 time.
Federal Air in Johannesburg if you have Caravan time and really want to scare yourself (game farm strips).
If it's humanitarian work you want, then contact AirServ in the US or, if you have at least 500 hours, send your CV to Naturelink in Pretoria (att Daan Brandt, Chief Pilot) who do their flying in Africa and the Middle East.
BTW, it's not nearly as glamorous as it looks on TV and you don't see the guys with RPGs and AK47s just waiting in the bushes....