A full list of official archive mirrors can now be found in launchpad.
Each mirror listed in launchpad is verified at least every 24 hours. You are recommended to select a mirror marked “Up to date” that is located geographicly near you. If you are unsure which mirror to select the best option is [iso-country-code].archive.ubuntu.com where iso-country-code is the two character country abreviation of a country near you. For example, if you live in the United States you could choose us.archive.ubuntu.com
See also :