Try:
protected void fmFrom_SelectedIndexChanged(object sender, EventArgs e)
{
for (int i = 0; i < rateRepeater.Items.Count; i++)
{
DropDownList from = (DropDownList)rateRepeater.Items[i].FindControl("fmFrom");
((TextBox)FindControl("TextBox" + (i + 1))).Text = from.SelectedValue.ToString();
}
}