Maybe someone will have an answer about finding the credential in Jo'burg (not sure if there's a CSJ office there), but to be honest most people just get their credential when they arrive in France or Spain: you can get one pretty easily from the church, tourist/camino office, or even the hostel, for a couple of Euros.
The South African group is one of the most active and organized.
They can help you with every aspect of your Camino. A great group of new and experienced Pilgrims.
Hopefully one of the members will reply to you here in this thread.
Hi Alton, I attended one of the Confraternity's workshops at the Botanical Gardens in Pretoria, and bought a credential there. They did not ask that I become a member. It's a really nice credential, with many pages, so you can collect LOTS of interesting stamps along the Way! Jill