Where can I buy an iTunes card in Vietnam?

I am presently residing in VietNam and Apple makes me buy only through the Vietnam Appstore and iTunes Store. This is OK with me as I don't see much difference at least for the Apps I am interested in. The problem is that I can't download any paid Apps because I had to reset my Apple ID for no credit card. This is because a US card may not be used in the VN store and I do not have a Vietnamese Credit card.

I was told by an Apple support agent that I could use an iTunes card instead. I tried that with a card that I got from the US but I could not add it. The system told me the card was only good in the US store.

I woiuld like to buy an iTunes card that will work in Vietnam but I really don't know where. Future World which is the Apple Authorized Reseller in Ho Chi Minh City do not carry them. If anyone had them, I would think they would but they refered me to one of the cell phone providers and that turned up negative.

Apple is really the loser here as I would like to purchase Mountain Lion but I can't do it.

Can anyone help me on this? Thanks.