On a Debian-based system, including Ubuntu, how can one tell which repository a package will be downloaded from, without actually beginning the download? aptitude show and apt-cache info will show the section (e.g., metapackage, base, graphics), but not the repository to which a package belongs (e.g., http://ppa.launchpad.net/mactel-support/ppa/ubuntu or http://us.archive.ubuntu.com/ubuntu/).

When installing the package, the actual repository appears during the download (it is printed out in the "downloading from ..." output from apt and similar programs), but how can one obtain information on the repository containing the package (or a specific version of a package) without downloading and installing it first?

Additionally, how can one determine the source repository for a package that is already installed?