Long time..... but bad and great news.
I found 2 Cape guns but they are not for sale, they are both in the Ugie / Maclear area in the hands of farmers, not willing to give them up.
In South Africa I notice the farmers refer to them as either "side by sides" or "over and unders". Both the types that I came across where family heirlooms that are still firing perfectly, got to shoot the "over and under".
They are both from the 1800's, there are quite a few around if it is what you are looking for.
On the good news, there's a site in South Africa that often has these cape guns for sale.www.clasicarms.co.za
(keep watching them their stock changes every month or so)
Just a word of warning make sure that you know exactly what you are after as it seems that we, South Africans tend to have either a "comman" name that anything that looks like that will go into that group or it is reffered to in Afrikaans.
You are proberly looking for a muzzle loader side by side Cape Gun. (the ones I saw were muzzleloaders). In that case when you speak to anyone in South Africa that does not understand the term "muzzle loader" refer to it as a Voorleier(Afrikaans term)
Any questions or if you need help, just shout, hope that helped