Woolworths (nothing to do with the firm that went under in the UK) is a fantastic source of food if you are self catering in SA. Some products are identical to M&S foods you'd get at home (at rather more competitive prices). Look on the website for store locations before you go - we visited the Cape Town stores but there are others. There are some interesting variations on a theme to take account of local cuisine as well as the usual favourites (the latter very useful if travelling with kids.)
Updates
Woolworths South Africa also offers online shopping and convenient home delivery in most of the major cities in South Africa. Check out the website for details.
Is this your business?