In order to find a control on a page, which my be nested in a repeater or another control, this method might be useful

private Control FindNestedControl(Control parent, string controlid)
{
    Control c = parent.FindControl(controlid);
    if (c != null)
        return c;
    else
    {
        foreach (Control child in parent.Controls)
        {
            c = FindNestedControl(child, controlid);
            if (c != null)
                return c;
        }
    }
    return null;
}

Which can be called with Page object as the initial Control, and then it iterates all the controls on the page untill it finds the correct one.