As I know, both of the bus stations at Shanghai Railway Station or Shanghai South Railway Station supply buses to Zhangjiagang. But, the former station has much more.

It is not suggested that you take the Maglve, which is expensive and a little complicated to get to the bus stations. However, the shuttle buses could drive you there directly:
To Shanghai Railway Station: please take the shuttle bus Line 5
To Shanghai South Railway Station: please take the shuttle bus Line 7