Yeah flysaa.com does insist on SA credit cards for domestic special fares.
However
Some of the other carriers in SA don't (kulula.com / 1time etc.) .. I think you can purchase tickets (e-ticket) from them using any credit card
Alternatively flysaa.com does allow you too book the tickets and have them issued at an SAA ticket office in SA within 48hrs .. you could probably do this just before you travel and then pick up the ticket in SA paying with cash (or probalby even your non SA credit card) .. Risk of course is that the flights might be sold out / at higher fares .. depends on when you are travelling.