It depends where you are in China. Different provinces have different stores. In Hebei you can top up at any 'On Time' convenience store or 'Meishilin' supermarkets.

In Kunming you can top up at many of the street news stand, but there are none of the a fore mentioned shops in Kunming.

Just about any convenience store can top it up, same with street vendors, just give them your correct phone. I gave the wong number once, no wefunds.

You can also top up your phone online and via tele-banking but you will definitely need the help of a Chinese person to set this up for you. Usually newspaper stands also sell top-up cards. Just look out for places that have a blue sign or that offer public phones - these usually also sell top-up cards. My local cigarette and baijiu vendor also sells them so it may be worth learning the phrase "ni you shouji chongzhi ka ma?" and slowly scoping out all the 'hidden' top-up sellers.