What is the abbreviation for 'South Africa'?

It varies.

RSA - Republic of South Africa is best.

But SA is often used, rather like US is used for USA or UK is used for UKOGBANI (United Kingdom of Great Britain and Northern Ireland) :-)

On the Internet it is .za This is from the Dutch "Zuid Afrika"

Aeroplane registrations are ZS

Radio callsigns are ZS and ZR and a few others.