You might want to try one of the unofficial Apple stores in Chengdu, but iTunes gift cards in China are notoriously not genuine. There’s an enormous black market of iTunes gift cards which I know a lot about. Here’s how it works in summary:
1. Scammers purchase iTunes gift cards with stolen credit card information obtained from overseas
2. They re-sell the gift cards to buyers online at 100% profit
So if you see an iTunes gift card that is cheaper than it should be, this is what’s happening behind the scenes.