How to populate a DropDownList based on another DropDownList selected value and display all records in datagridview

Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
Dim cn As New SqlConnection("Data Source=.\INSTANCE;initial catalog=stamp;user=sa;password=gariahat")
'Dim da As New SqlDataAdapter
Dim cmd As New SqlCommand
Dim ds As New DataSet
Dim dt As DataTable




cmd.Connection = cn
cn.Open()

Dim da As New SqlDataAdapter("select * from agent", cn)
dt = New DataTable

ds = New DataSet
da.Fill(ds, "agent")

For i As Integer = 0 To ds.Tables("agent").Rows.Count - 1
If ComboBox1.SelectedItem = ds.Tables("agent").Rows(i).Item("broker_id").ToString() Then

da = New SqlDataAdapter("select a.broker_id, a.name,a.place,a.date,a.year,b.name,c.item_id,c.name,d.amt from agent as a,buyer as b,item as c,trans as d where a.broker_id=d.broker_id and b.buyer_id=d.buyer_id and c.item_id=d.item_id", cn)
dt = New DataTable
da.Fill(dt)
DataGridView1.DataSource = dt
' cn.Close()

End If
Next

cn.Close()


End Sub
End Class