Using multiple dropdownlist in gridview control

protected void drplocation_selected(object sender,EventArgs e)
    {
        DropDownList ddl = (DropDownList)sender;
        string locationid = ddl.SelectedValue.ToString();
    }
In this event i need to populate drpstore based on the location selected. 

In the above event
How do i access the other dropdownlist which is in the next column of the same row?