The problem you are encountering has nothing to do with your IP address (from my past experience).
It is more to do with the websites that you are trying to buy from as using PayPal as their acceptor of payments. Yes, you will not be making a PayPal payment but because PayPal will be processing the Visa and/or Master Card payment, they do not recognise payments from Zimbabwe which is currently under "sanctions".
You can use http://torproject.org
to hide your IP address (or rather, appear to be in another country), and let us know if you have any sucess.