Remove the brackets to start with:
String selQuery = "SELECT Bus_Route,Cost FROM MapDataImage WHERE Source='" + Source_Box.Text + "' AND Destination='" + Distance_Box.Text + "'";
Then stop doing SQL like that! Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead:
String selQuery = "SELECT Bus_Route,Cost FROM MapDataImage WHERE Source=@SC AND Destination=@DS";
scmd1 = new SqlCommand(selQuery, conn2);
scmd1.Parameters.AddWithValue("@SC", Source_Box.Text);
scmd1.Parameters.AddWithValue("@DS", Distance_Box.Text);
SqlDataReader sqldread2 = scmd1.ExecuteReader();
if (sqldread2.Read())
{
RouteDetails.Text = sqldread2["Bus_Route" + " Cost "].ToString();
}