Hmm.... locations are one of the toughest things on a low budget. Ever.
Your first and best attempt would be locations companies that broker the locations. Now just remember they work on a comission - usually a third of the location rate - so they are not really working for you, but themselves and the properties they rep.
The only African thing I can think of at all that I have come across is an African village at Blue Cloud Ranch in Santa Clarita; not sure if it's still standing. A quick google image search of rural Zimbabwe farms is not encouraging- nothing like California at all...
What bout a door / entrance set piece with a CGI house around it as an establishing shot, at the location that looks like your imagined farm but lacking a house ? Often times, selling the illusion once or twice works better than maintaining it. Interiors on a stage, and exteriors all looking away from the farmhouse ?
Proof again, filmmaking is really easy... just not easy. Locations are tough particularly in L.A. becasue no one likes fillmaking per se other than filmmakers, so they want to get paid, not be helpful.
Something simple like a treehouse can be challenging when you get specific about it's environment and size - I know where every treehouse in Los Angeles is after a project last year; we secured a superb one at a pretty fair price. after almost spending more to build one in a tree that was just OK... Same thing with a big rolling lawn with nothing in the horizon- I just booked that, but it took a week fulltime and about $4500 for a half day... the one day last week it was clear and sunny, which if not would have killed the shoot.