Hey I have spent ages looking for this all over, the best I have been able to find is at April's or HGB supermarkets (in the village.) No where has proper American cornmeal (like for cornbread) but the usually have corn flour in the baking sections and then you can also by a similar coarse ground corn meal (sold in the same section) and if you mix them together you can come out with reasonable cornmeal mix

The bulk section of most Chinese grocery stores have cornmeal. For example the JingKeLong down the street from ChaoYang Park South Gate.

Cornflour is similarly widely available, though I notice the quality varies greatly.

Don't know anything specific to the vast wasteland that is Shunyi, but there has to be something similar in the Chinese parts of town.