You'd have to find a company that will ship to your country and if the prices there are higher due to taxes - import duties etc. those will also be added by the company you purchase from (in order to do business in other countries companies have to follow the legal methods of shipping products into that country) - so the price you see advertised in another country probably will not be what you wind up paying. Do you know anyone you trust that lives in the US that you could have the company ship the product to and have them forward it to you ? (doing it that way might be cheaper - especially if they have a system to test the card on to ensure it is working before sending it on to you so any DOA product could be RMA'd prior to sending it to you) -- but as you mention warranty would be a bit tougher since you'd have to mail it to them and they RMA it then reship it back to you after it was repaired\replaced -- so shipping fees would add up.
You may not find an online company that will ship overseas. Where are you anyway. You may be able to find something on E-bay where the seller will ship international. An online company that may do shipping is Amazon.com.
I think that Amazon might be your best option as I have come across a few posts that people were using Amazon in thier purchases for building a new Pc and they were in different countries around the world , ao you may want to do a search for Amazon.South Africa.
Ps. Don't forget to select the best answer if you were satisfied with the help you recieved with your post question.