The sushi kit could be purchase at these supermarkets (at the booth of Japanese or Korean food), just find one close to where you are.

Hi, Mrs. Maria
You can visit Hongqiao Friendship shoppong center, because around that area there are many japanese live there. I am pretty sure you can find the sushi stuff.

here is address:No.6,ZUNYI ROAD(S).SHANGHAI,CHINA

You can have a look their supermarket, there are sushi vinegar and japanese rice, you also can get some fresh salmon there.

here is their web address: http://www.friendship-hongqiao.com/english/index.asp

Oh , i forgot there are other place you also can find sushi making kit

Jiu Guang City Plaza

Address:1618,Nanjing Road(W), Shanghai

However it is bit expensive compare with local supermarket. i am pretty sure not may locals buying
food in there. But you can find many fruits and veg across abroad like Avocado (per RMB 40)

You can take off the tube at jinan si 静安寺 (line 2) on your right side, just walking distance.(8mins)