Ubuntu 12.04 server with mirrored packet repository

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http://serverfault.com – I have an 12.04 ubuntu server with a modified source.list My server is a VM at an hosting company, they are the one providing a mirror for repo. The mirrors does not mirror all packets, and i am unable to install image magick. Weirdly apt-get can list the necessary packets for the install but fails at download, i wonder why the mirrors can give the packets list but does not host them. I'd like to add the officials ubuntu packet repo as a "fallback" when packets are not present on the mirror server (they are fast). I cannot find sources.list for precise pangolin server, where can i find it? H (HowTos)