Yes, you'll be able to buy an adaptor at JNB. You won't be the first non-SA passenger to need one.

If near a Clicks branch (similar to Walgreens/CVS/Duane Reade but brighter) or Pick'n'Pay supermarket, you'll probably find them there at lower prices than at the airport. My brother- and sister-in law bought some at Clicks V&A Waterfront earlier this year - theirs were to convert German Schuko plugs.

Invariably, hotels and B&Bs will lend you one, though I do understand you would prefer your own.