On sweater street and Wu Yi Liu there is a store called Green Tree (I think). Anyways, the store front is green, and I believe there is a tree on the sign. I think these stores get name brand irregular items from factors and sell them at a discounted price. I am a big guy myself, and I have been able to find many nice shirts, sweaters and pants. Also, the price is very reasonable. I would go to these places first.

You could try Walmart, you might be able to wear their XXXL shirts, and they defo had some 36 pants, there's a chance they might sometimes have some 38s

Probably right about Walmart, tho I hope you can find someplace else.

A couple of Gap stores have opened in the new malls. Didn't go in, so not sure if they have +sizes, but it's American clothing in theory. Some department stores online sites will ship overseas, like Macy's.

Does H&M in China carry +size?

15 shoes, you're probably sol, but the sporting stores around the stadiums maybe.