Upvote:3
If you "have to make sure that [you] can purchase it with certainty", I would highly recommend that you buy a SIM card at the airport.
In general, non-Chinese credit cards are rarely accepted within China, even online. They are usually only accepted at high-end stores or stores that cater to foreigners.
A bigger problem you may have is trying to get a SIM card (outside of an airport) without a National Identity Card. The stores that I have tried in Chengdu and Beijing will not sell you one without an ID card. As a foreign citizen living in China, I am currently using a SIM card that is tied to a friend's ID. I believe there is in concept, a way to use a passport in place of the ID, but the stores I have tried did not accept it.
Another problem you will have is trying to get a SIM card if you don't speak Chinese.
==== update ====
If you need to buy a SIM card/phone plan while in China, you will need to have your passport with you. Also, China Unicom is your best bet in terms of service provider.
The cases I've mentioned above where the stores refuse to sell a plan/SIM card to foreigners were China Mobile/Telecom chains.