I'm no expert either but the .deb files are there in your distro. They
are well hidden, but look for them in /var/cache/apt/archives. Unless
you have deleted the installation files, either accidentally or on
purpose that's where they are always stored. I checked my Mint machine
and they are in the same place there and I'm sure in Ubuntu and
Kubuntu. They aren't "hidden" so you should be able to see them
easily.