If your combobox is data bounded than try to run this code through another control event. I mean Insert a button into Form and run your code in click event of that button.Best way to do that-
1.Convert your combobox.selectedvalue into the data type of column data type in database. ( I assume it is integer type)
2.into the click event of button-
Dim supNo as integer=CInt(cmbSupplierName.SelectedValue)
Dim dsvalue As DataSet
dsvalue = obj.GetView("Select p.PO_no,p.Date,s.Name from PO_Header p, SupplierMaster s Where p.Sup_no = " & supNo & ";" )
DataGridView1.DataSource = dsvalue.Tables(0).DefaultView