If you want to display data in a DataGridView, don't use a DataReader - use a DataTable and a DataAdapter instead:
string getdata = "SELECT s.deal_amnt, s.paid_amnt, m.due_amnt, m.veh_model, m.model_year FROM tbl_vehicle_sale s INNER JOIN tbl_vehicle_pur_Mast m ON s.veh_no = m.veh_no";
using (SqlDataAdapter da = new SqlDataAdapter(getdata, cn))
{
DataTable dt = new DataTable();
da.Fill(dt);
dataGridView1.DataSource = dt;
}
If you need a WHERE clause just add it, and add a parameter:
string getdata = "SELECT s.deal_amnt, s.paid_amnt, m.due_amnt, m.veh_model, m.model_year FROM tbl_vehicle_sale s INNER JOIN tbl_vehicle_pur_Mast m ON s.veh_no = m.veh_no& WHERE s.deal_amnt = @AMNT;quot;;
using (SqlDataAdapter da = new SqlDataAdapter(getdata, cn))
{
da.SelectCommand.Parameters.AddWithValue("@AMNT", amountToFilterBy);
DataTable dt = new DataTable();
da.Fill(dt);
dataGridView1.DataSource = dt;
}