Ubuntu – “apt-get source” cannot find package but “apt-get install” and “apt-get cache” can find it

I'm trying to download the source of a package but when I run

root@proxy:~# apt-get source squid3
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Can not find version '3.3.8-1.1' of package 'squid3'
E: Unable to find a source package for squid3

It cannot be found. But when I run apt-cache search squid3 it finds it.

root@proxy:~# apt-cache search squid3
squid-cgi - Full featured Web Proxy cache (HTTP proxy) - control CGI
squid3 - Full featured Web Proxy cache (HTTP proxy)
squid3-common - Full featured Web Proxy cache (HTTP proxy) - common files
squid3-dbg - Full featured Web Proxy cache (HTTP proxy) - Debug symbols
squidclient - Full featured Web Proxy cache (HTTP proxy) - control utility

It works fine with apt-get install squid3 as well. I am using Ubuntu 11.04 server and this is my /etc/apt/sources.list

deb http://old-releases.ubuntu.com/ubuntu natty main restricted universe
deb http://old-releases.ubuntu.com/ubuntu natty-updates main restricted universe
deb http://old-releases.ubuntu.com/ubuntu natty-security main restricted universe
deb http://archive.canonical.com/ubuntu natty partner
deb http://ftp.us.debian.org/debian sid main
deb http://packages.dotdeb.org stable all
deb-src http://packages.dotdeb.org stable all
deb http://archive.ubuntu.com/ubuntu natty universe
deb-src http://archive.ubuntu.com/ubuntu natty universe

I've apt-get updated many times. I have tried many different debs and have not found this error any where else.