Comment 7 for bug 820383

Just to show a workaround. It's really *UGLY* but worked for me:

1.- Download manually
http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_xx.x.x.x.orig.tar.gz

2.- Make you root (sudo -i, su -, etc)

3.- Make a backup of /usr/bin/update-notifier/package-data-downloader:
cp /usr/bin/update-notifier/package-data-downloader /usr/bin/update-notifier/package-data-downloader.bak

4.- Edit /usr/bin/update-notifier/package-data-downloader and replace this line:

dest_file = urllib.urlretrieve(files[i])[0]

by this one:

dest_file = urllib.urlretrieve("/path/to/your/file/adobe-flashplugin_xx.x.x.x.orig.tar.gz")[0]

5.- Reinstall flashplugin-installer package:
apt-get install --reinstall flashplugin-installer

6.- Recover the original package-data-downloader file:
mv /usr/bin/update-notifier/package-data-downloader.bak /usr/bin/update-notifier/package-data-downloader