I would assume that getting a replacement battery locally would be a mission. For example the battery replacement kit mentioned in the previous post, the company does not ship to South Africa. Also, (I may be wrong about this), I think that shipping companies are not allowed to send batteries by airfreight anymore. I would rather browse the classifieds section, there was a number of Edge 500s up for sale at very reasonable prices. I purchased mine here of the Hub's classified section a number of years ago, and it's still going strong with really good battery life.
Another option would be to see if you cannot hack a battery intended for a cell-phone, and with a bit of soldering you may just come right.