I'm in South Africa. I must admit that I've had very little luck finding SA ponies (or any ponies at all!) anywhere other than online. There are a few charity shops, but I haven't had too much luck in finding any ponies there: I've only ever found 1! They tend to have mainly clothes and books.
I've spoken to some seller from the SA auction site and they said that they've mainly found ponies at markets. Any they say they try to get there just as it opens, as the ponies go fast! PM me and I can give you more details about markets that I know about.
The good news is that you do manage to find ponies, a fairly large percent of them seem to be SA ponies!