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where can i buy them?

I've looked online and can only find "collectable" South african stamps. Your best bet would be to use "International Reply Coupons".
By viewing the Universal Postal Union's website it is shown that South Africa have the facilities to issue and process reply coupons so your recipient would be able to change them into stamps to send a letter back to you. [link]

You can purchase these "coupons" from your local post office and can find out more information about them here.

Good luck with your collecting