Shotgun ammunition in Southern Africa of any sort is easy to acquire but may take some effort and investigation. Two years ago I ordered a flat of 2-1/2" twelve gauge cartrides with 30 gram (1-1/16) load from Rosenthals in Windhoek. They brought it up from Rosenthals in Cape Town. Suggest you call or use the website contact form for The Powder Keg in Joburg and see if they sell the .410 ammo you want, can get it, or know where you can order it. If they can get it, once off the plane, take your Temporary Firearms Import Permit and go to the shop and pick up a flat or so...

Regards