国内一般都保留最近几个安装源，旧版的安装源如下：
http://old-releases.ubuntu.com/ubuntu/
我的是15.04，版本代号为vivid，sources.list如下，其他版本的把vivid替换为对应的版本代号就可以了：
deb http://old-releases.ubuntu.com/ubuntu/ vivid main restricted universe multiverse deb http://old-releases.ubuntu.com/ubuntu/ vivid-security main restricted universe multiverse deb http://old-releases.ubuntu.com/ubuntu/ vivid-updates main restricted universe multiverse deb http://old-releases.ubuntu.com/ubuntu/ vivid-proposed main restricted universe multiverse deb http://old-releases.ubuntu.com/ubuntu/ vivid-backports main restricted universe multiverse deb-src http://old-releases.ubuntu.com/ubuntu/ vivid main restricted universe multiverse deb-src http://old-releases.ubuntu.com/ubuntu/ vivid-security main restricted universe multiverse deb-src http://old-releases.ubuntu.com/ubuntu/ vivid-updates main restricted universe multiverse deb-src http://old-releases.ubuntu.com/ubuntu/ vivid-proposed main restricted universe multiverse deb-src http://old-releases.ubuntu.com/ubuntu/ vivid-backports main restricted universe multiverse