Comparing string arrays
I would do this:

protected void Button1_Click(object sender, EventArgs e)
{              
    string[] str1 = new string[] {"87002", "87001", "87005"};
    string[] str2 = new string[] {"97002", "97003", "97004"};

    if (str1.Contains(TextBox1.Text))
        Response.Redirect("Page1.aspx");

    if (str2.Contains(TextBox1.Text))
        Response.Redirect("Page2.aspx");

    Response.Redirect("Unknown.aspx");
}


You don't need the ToString() calls - TextBox.Text should already be a string.