Ubuntu/Debian: Show list of available versions of a specific package

How can I list all available versions of specific package?

I know with

apt-get install myPackage=1.2.3 

a specific version could be installed.
And with

apt-show-versions -a myPackage 

I would get a list of versions that are known by the system.

But how getting a list of all available versions.
I think that isn't possible using the apt tools because they are
restricted to configured repositories.

So what is the way to go? Some web-repositories? What is the recommondation for Ubuntu 8.04?