A major company in South Africa has offered us a partnership. I need to fly there personally to inspect the production of the products we require, assess their quality, and possibly sign supply contracts. I will need transportation to get around the sites. I know that you can rent a car at Johannesburg Airport, but the prices are quite high. Are there other car rental options available in the country?

Yes, renting at the airport is not worth it. They have many requirements, the deposit is substantial, and the prices are very high. There are cheaper options. For example, you can check the cheapest car rental options in South Africa here: https://getrentacar.com/en-US/south-africa . They can even have the car delivered to the airport at the agreed time and hand over the keys there.