I would suggest the following on-line sources to start with:
National Archives
familysearch - once you have the information from the archives you can then look at probate files / death notices. You could also find marriage and baptismal information on this site.
eggsa.org - Photographs of tombstones and some extracts from various newspapers
Birth certificates are not available on line in South Africa - only baptismal certificates. You would have to apply to the department of Home Affairs for these.
Another way would be to post the details of the people you are looking for and see if rootchat members can be of assistance.
Hope this helps.