Please try some thing like this
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
Control[] ctrl = this.Controls.Find(comboBox1.SelectedItem.ToString(),true);
if (ctrl.Length > 0)
{
DataGridView dgv = (DataGridView)ctrl[0];
textBox1.Text = dgv.Rows[dgv.Rows.Count - 1].Cells[0].Value.ToString();
}
}