Where are .deb files located?

By default, apt stores the .deb files in /var/cache/apt/archives. If you can't find them here, you or someone else or something else(a software like Ubuntu tweak) may have run a clean-up command like sudo apt-get clean to clear up the .deb files. However, you can manually search all .deb files in your system by doing the following(may require superuser privileges):

sudo updatedb

This will update locate's database to search for newly created/removed files, this may take a while depending upon the difference in time when the last time updatedb was run(it is usually run as a cron job daily) and then

locate *.deb

which will search your / folder recursively for all files ending with .deb.