There's a few options here.
Tracks4Africa needs a license, but it's not very expensive and is very good for areas where you don't have any coverage from other GPS maps.
There are various options.
You can download or buy on an SD card.
And buy a one-off or get a lifetime subscription.
The one-off is ZAR 790 (about USD100).
Welcome to Tracks4Africa
Then there's Garmap (not GARMIN).
Garmap South Africa Garmin GPS Navigation device - maps software tool
I think it's better than Garmin South Africa.
And then there's the other options already mentioned.
What you should be able to do is put in an SD card to your PC and download a map to that.
Or to your computer and then to the SD card.
So you can have several map systems, each on a different SD card.
You obviously need to label them...:-}
So you just swap them into your GPS when you want that map system.
I do that.
When you look at time and petrol and other costs, paying for two GPS map systems is peanuts.
If you've got an SD-type GPS and some way to plug an SD card into your PC there is no special format.
The Garmin reads the standard SD format.
If you're in South Africa why don't you just stop by an outdoors store that sells Garmap and they'll help you set it up.
Or call the local support number for Garmaps or Tacks4Africa.
They are both helpful groups of people.