You would have to find a US reseller that would accept a non-US credit card. Apple will not. You can try Best Buy. They say that they'll accept non-US credit cards as long as the delivery address will be in the US.
but at least one other user said that they would not accept an order for an iTunes gift card. You can try, though. If they refuse, then purchasing a US iTunes card from outside the country will most likely not be possible.