Where to find older source&binary package versions?

Ubuntu uses Launchpad to host all its packages specifically here:

https://launchpad.net/ubuntu

From here you can find the source code to any package simply by searching

For example by searching for for acpid you are taken here

https://launchpad.net/ubuntu/+source/acpid

from here you can View the full publishing history, or View full change log to find all releases and select the one you want.

In many cases you will also be able to find binaries. However I have seen examples where the binaries are missing. I'm not sure why but I presume this is when the main truck has changed more than once between Ubuntu releases.

The developer may have updated the source from upstream project for the first upstream change, tested it and published the source then immediately updated the with the second update and published again with the current upstream release.