I think what you are saying is that when the user selects the supplier from the drop down, you'd like another field to have the value of the supplier account number - is that correct?
So, you can use rules or default values for this.
Let's say you use a default value.
On the field that you want to have the account number in, you'd right click the field in the Main Data Source in the data source task pane.
You'd set the default value - clicking the fx button to open the forumula editor. Use the insert field or group button and navigate to the account number field in your secondary data source. Click the Filter Data button. The filter will have 3 drop downs - the first drop down you'll select the Supplier field from your SharePoint list. The middle drop down you'll leave as "is equal to". For the final drop down, select the main data source node that is bound to the drop down where the user selects the supplier.
Hope that helps.