
From 1 April 2015 SA Express will fly from Cape Town International Airport to Pilanesberg Airport twice a week (Mondays and Fridays).
Alternatively you can do the following:
hire a car at Johannesburg Airport and drive to Sun City (some 2 hours)
There aren't even any direct intercity buses from Cape Town to Sun City - you can catch a cheap intercity bus from Cape Town to Johannesburg Park Station (with Intercape the cheapest options are Budgetliner and Big Sky Intercity coaches) and then from Johannesburg to Rustenburg, then take a taxi from Rustenburg to Sun City.