Hi seasoned traveller. You can certainly buy China SIM cards online. If you buy through Amazon it is a little more expensive than buying elsewhere online, but through Amazon the seller will activate the SIM remotely, so you know the number before you leave.
For other online you must wait until you land, but it is easy to then activate. The company that I used last time, November, had a China & a HK number on one SIM, which was useful for me.
Very useful to know FF – thank you for this info – I will look into it.
For the record, Once my credit with WorldSim runs out, I will not be spending any more with them.