Yiwu is not far from Shanghai. No need to take a plane to go there. Bus and train both are good. You can take a train at Shanghai Railway Station or Shanghai South Railway Station to go to Yiwu. Shanghai South Railway Station provides more trains to Yiwu. I tbink it's better for you.

As for the bus, you may read the following page. It tells you the major bus stations in Shanghai.
Ps: It takes about 4 hours from Shanghai to Yiwu by train or by bus. Wish you a good trip.