Preferably in monthly or weekly denomination, for at least 20 years. Preferably in Excel format, but csv, or other format is fine too. I try to google but couldn't find anything, maybe I just wasn't doing it right.
Try http://www.dukascopy.com/swiss/english/marketwatch/historical/ You can download it as a csv file. Look for XAUUSD (Gold) & XAGUSD(Silver) under the list of instruments.