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In the park, at Skukuza, there is an Avis rent a car. Since some tourists fly into Skukuza, it's convenient for this to exist.
Prices will vary depending on time of year and your requirements, and we don't tend to permit price-shopping answers on here as things change constantly, but have a look and see what you can find.
From my experience, a vehicle like a Toyota Condor was fantastic.