In the last five years or so, almost all those small stalls selling SIMs -- the ones with boards outside showing the available numbers and prices -- seem to have disappeared, leaving you with telco shops as the only real alternative. Keep in mind that often only the larger shops can register passports. Also remember that if there are problems down the road, like needing to replace or upgrade your SIM, you'll need to handle it in the same province, and sometimes the same city, where you bought the card.

Bear in mind too that numbers get re-used, and there's always a risk that your new number won't work someplace because it's already been registered. So register everywhere you want to register while you're in China and can deal with any problems.

Finally, both WeChat and Alipay have different versions of their apps that seem to get installed automatically depending on your phone settings and where you download them. To avoid problems, do a fresh download and install of the apps when you get to China and then set up fresh accounts using your new number (don't try to change exisiting accounts to a Mainland number).

(I've heard that Unicom can be fussy if you don't cancel a prepaid number and just let it run out of money and die. CM doesn't seem fussy in that way.)