The Apple stores sell lots of accessories which are third party brands.

I found a case and magnetic cover for less than 200 rmb at the underground mall between the metro exit and the central square at Shanghai Railway Station. If you have a Chinese friend who can go in and bargain for you, your price will be lower. The cases you buy at Xujiahui and elsewhere are all the same quality but priced 2 to 300 more, especially if you're a laowai.