Try any Canada consulte. It is hard.

It is very hard for B1/B2 holders to get visas, esp. those with less internatioanl travel history. There was once explicitly stated on Canadian Embassy website that Chinese B1/B2 holders cannot apply for visitor visa in US. Just like Schengen visa and most other european visas, B1/B2 holders are asked to go back to China and apply ther.
But you can definitely give it a try.It costs $65(Can$75) per application for single entry visa.