using System.Collections;

...

for (int i = 0; i < lwFiles.ColumnHeaderCollection.Count; i++)

var index = lwFiles.Columns.Cast<ColumnHeader>().Select((n, i) => n.Text == "MyHeader" ? i : -1).First(i => i >= 0);

			var lw = new ListView();
			lw.Columns.Add(new ColumnHeader { Text = "123" });
			lw.Columns.Add(new ColumnHeader { Text = "456" });
			lw.Columns.Add(new ColumnHeader { Text = "789" });
			var index = lw.Columns.Cast<ColumnHeader>().Select((n, i) => n.Text == "456" ? i : -1).First(i => i > 0);
			MessageBox.Show(index.ToString());

        private void lwScoresFiles_SelectedIndexChanged(object sender, EventArgs e)
        {
            ListView LW = (sender as ListView);
            var selectedItems = (sender as ListView).SelectedItems;
            int nRow = 0;
            foreach (ListViewItem item in selectedItems)
            {
                nRow = Int32.Parse(item.SubItems[0].Text);
                DBC_FILEID = Int32.Parse(item.SubItems[item.SubItems.Count - 1].Text);
            }
            DB_ShowProcess(lbDBProcessText, "Запись " + nRow.ToString() + " из "+ DBC_FILESCOUNT.ToString() + ", ID = " + DBC_FILEID.ToString(),  false);
        }

        private void lwScoresFiles_SelectedIndexChanged(object sender, EventArgs e)
        {
            var item = (sender as ListView).SelectedItems[0];
            DBC_FILEID = Int32.Parse(item.SubItems[item.SubItems.Count - 1].Text);
            DB_ShowProcess(lbDBProcessText, $"Запись {item.SubItems[0].Text} из {DBC_FILESCOUNT}, ID = {DBC_FILEID}",  false);
        }