Dixie,
In South Africa (and Windhoek, Namibia too from what I hear) you can buy most of the "std" type calibers you'd buy in the smaller US centers.
I know that apparently (from some or other study done recently) 30-06 and then .308 are our most popular hunting calibers in that size range.
.270 and .243 (not that you'd probably take one on safari) are very popular, as is 7X57 (often called 7mm Mauser here) and 7mm Rem Mag. .223 is widely used, although I don't view it as an antelope caliber aside from the tiny species.
.375 in the bigger stuff.
You'll likely get .300 Win Mag and .458 Win Mag.
If you know where to look you'll get most of the more popular stuff and, at the right dealers, many of the German / European calibers. You are not likely to be spoilt for choice with which brand of ammo and style of bullet is available.
Bear in mind that after our legislation changes very few "new" rifles have been purchased the last few years, meaning that the newer calibers aren't widely carried by the dealers. The enthusiasts are buying the newer stuff and most are reloaders.
If you know who to ask and where to look you'll find most. I suspect your PH will be able to guide you and locate what you need, if it's not too out of the ordinary.